diff --git a/spec/AuthDataUniqueIndex.spec.js b/spec/AuthDataUniqueIndex.spec.js
new file mode 100644
index 000000000..d975a4220
--- /dev/null
+++ b/spec/AuthDataUniqueIndex.spec.js
@@ -0,0 +1,210 @@
+'use strict';
+
+const request = require('../lib/request');
+const Config = require('../lib/Config');
+
+describe('AuthData Unique Index', () => {
+  const fakeAuthProvider = {
+    validateAppId: () => Promise.resolve(),
+    validateAuthData: () => Promise.resolve(),
+  };
+
+  beforeEach(async () => {
+    await reconfigureServer({ auth: { fakeAuthProvider } });
+  });
+
+  it('should prevent concurrent signups with the same authData from creating duplicate users', async () => {
+    const authData = { fakeAuthProvider: { id: 'duplicate-test-id', token: 'token1' } };
+
+    // Fire multiple concurrent signup requests with the same authData
+    const concurrentRequests = Array.from({ length: 5 }, () =>
+      request({
+        method: 'POST',
+        headers: {
+          'X-Parse-Application-Id': 'test',
+          'X-Parse-REST-API-Key': 'rest',
+          'Content-Type': 'application/json',
+        },
+        url: 'http://localhost:8378/1/users',
+        body: { authData },
+      }).then(
+        response => ({ success: true, data: response.data }),
+        error => ({ success: false, error: error.data || error.message })
+      )
+    );
+
+    const results = await Promise.all(concurrentRequests);
+    const successes = results.filter(r => r.success);
+    const failures = results.filter(r => !r.success);
+
+    // All should either succeed (returning the same user) or fail with "this auth is already used"
+    // The key invariant: only ONE unique objectId should exist
+    const uniqueObjectIds = new Set(successes.map(r => r.data.objectId));
+    expect(uniqueObjectIds.size).toBe(1);
+
+    // Failures should be "this auth is already used" errors
+    for (const failure of failures) {
+      expect(failure.error.code).toBe(208);
+      expect(failure.error.error).toBe('this auth is already used');
+    }
+
+    // Verify only one user exists in the database with this authData
+    const query = new Parse.Query('_User');
+    query.equalTo('authData.fakeAuthProvider.id', 'duplicate-test-id');
+    const users = await query.find({ useMasterKey: true });
+    expect(users.length).toBe(1);
+  });
+
+  it('should prevent concurrent signups via batch endpoint with same authData', async () => {
+    const authData = { fakeAuthProvider: { id: 'batch-race-test-id', token: 'token1' } };
+
+    const response = await request({
+      method: 'POST',
+      headers: {
+        'X-Parse-Application-Id': 'test',
+        'X-Parse-REST-API-Key': 'rest',
+        'Content-Type': 'application/json',
+      },
+      url: 'http://localhost:8378/1/batch',
+      body: {
+        requests: Array.from({ length: 3 }, () => ({
+          method: 'POST',
+          path: '/1/users',
+          body: { authData },
+        })),
+      },
+    });
+
+    const results = response.data;
+    const successes = results.filter(r => r.success);
+    const failures = results.filter(r => r.error);
+
+    // All successes should reference the same user
+    const uniqueObjectIds = new Set(successes.map(r => r.success.objectId));
+    expect(uniqueObjectIds.size).toBe(1);
+
+    // Failures should be "this auth is already used" errors
+    for (const failure of failures) {
+      expect(failure.error.code).toBe(208);
+      expect(failure.error.error).toBe('this auth is already used');
+    }
+
+    // Verify only one user exists in the database with this authData
+    const query = new Parse.Query('_User');
+    query.equalTo('authData.fakeAuthProvider.id', 'batch-race-test-id');
+    const users = await query.find({ useMasterKey: true });
+    expect(users.length).toBe(1);
+  });
+
+  it('should allow sequential signups with different authData IDs', async () => {
+    const user1 = await Parse.User.logInWith('fakeAuthProvider', {
+      authData: { id: 'user-id-1', token: 'token1' },
+    });
+    const user2 = await Parse.User.logInWith('fakeAuthProvider', {
+      authData: { id: 'user-id-2', token: 'token2' },
+    });
+
+    expect(user1.id).toBeDefined();
+    expect(user2.id).toBeDefined();
+    expect(user1.id).not.toBe(user2.id);
+  });
+
+  it('should still allow login with authData after successful signup', async () => {
+    const authPayload = { authData: { id: 'login-test-id', token: 'token1' } };
+
+    // Signup
+    const user1 = await Parse.User.logInWith('fakeAuthProvider', authPayload);
+    expect(user1.id).toBeDefined();
+
+    // Login again with same authData — should return same user
+    const user2 = await Parse.User.logInWith('fakeAuthProvider', authPayload);
+    expect(user2.id).toBe(user1.id);
+  });
+
+  it('should skip startup index creation when createIndexAuthDataUniqueness is false', async () => {
+    const config = Config.get('test');
+    const adapter = config.database.adapter;
+    const spy = spyOn(adapter, 'ensureAuthDataUniqueness').and.callThrough();
+
+    // Temporarily set the option to false
+    const originalOptions = config.database.options.databaseOptions;
+    config.database.options.databaseOptions = { createIndexAuthDataUniqueness: false };
+
+    await config.database.performInitialization();
+    expect(spy).not.toHaveBeenCalled();
+
+    // Restore original options
+    config.database.options.databaseOptions = originalOptions;
+  });
+
+  it('should handle calling ensureAuthDataUniqueness multiple times (idempotent)', async () => {
+    const config = Config.get('test');
+    const adapter = config.database.adapter;
+
+    // Both calls should succeed (index creation is idempotent)
+    await adapter.ensureAuthDataUniqueness('fakeAuthProvider');
+    await adapter.ensureAuthDataUniqueness('fakeAuthProvider');
+  });
+
+  it('should log warning when index creation fails due to existing duplicates', async () => {
+    const config = Config.get('test');
+    const adapter = config.database.adapter;
+
+    // Spy on the adapter to simulate a duplicate value error
+    spyOn(adapter, 'ensureAuthDataUniqueness').and.callFake(() => {
+      return Promise.reject(
+        new Parse.Error(Parse.Error.DUPLICATE_VALUE, 'duplicates exist')
+      );
+    });
+
+    const logSpy = spyOn(require('../lib/logger').logger, 'warn');
+
+    // Re-run performInitialization — should warn but not throw
+    await config.database.performInitialization();
+    expect(logSpy).toHaveBeenCalledWith(
+      jasmine.stringContaining('Unable to ensure uniqueness for auth data provider'),
+      jasmine.anything()
+    );
+  });
+
+  it('should prevent concurrent signups with same anonymous authData', async () => {
+    const anonymousId = 'anon-race-test-id';
+    const authData = { anonymous: { id: anonymousId } };
+
+    const concurrentRequests = Array.from({ length: 5 }, () =>
+      request({
+        method: 'POST',
+        headers: {
+          'X-Parse-Application-Id': 'test',
+          'X-Parse-REST-API-Key': 'rest',
+          'Content-Type': 'application/json',
+        },
+        url: 'http://localhost:8378/1/users',
+        body: { authData },
+      }).then(
+        response => ({ success: true, data: response.data }),
+        error => ({ success: false, error: error.data || error.message })
+      )
+    );
+
+    const results = await Promise.all(concurrentRequests);
+    const successes = results.filter(r => r.success);
+    const failures = results.filter(r => !r.success);
+
+    // All successes should reference the same user
+    const uniqueObjectIds = new Set(successes.map(r => r.data.objectId));
+    expect(uniqueObjectIds.size).toBe(1);
+
+    // Failures should be "this auth is already used" errors
+    for (const failure of failures) {
+      expect(failure.error.code).toBe(208);
+      expect(failure.error.error).toBe('this auth is already used');
+    }
+
+    // Verify only one user exists in the database with this authData
+    const query = new Parse.Query('_User');
+    query.equalTo('authData.anonymous.id', anonymousId);
+    const users = await query.find({ useMasterKey: true });
+    expect(users.length).toBe(1);
+  });
+});
diff --git a/spec/DatabaseController.spec.js b/spec/DatabaseController.spec.js
index b1ccc0d58..ac83c11dd 100644
--- a/spec/DatabaseController.spec.js
+++ b/spec/DatabaseController.spec.js
@@ -415,6 +415,11 @@ describe('DatabaseController', function () {
           email_1: { email: 1 },
           _email_verify_token: { _email_verify_token: 1 },
           _perishable_token: { _perishable_token: 1 },
+          _auth_data_custom_id: { '_auth_data_custom.id': 1 },
+          _auth_data_facebook_id: { '_auth_data_facebook.id': 1 },
+          _auth_data_myoauth_id: { '_auth_data_myoauth.id': 1 },
+          _auth_data_shortLivedAuth_id: { '_auth_data_shortLivedAuth.id': 1 },
+          _auth_data_anonymous_id: { '_auth_data_anonymous.id': 1 },
         });
       }
     );
@@ -441,6 +446,11 @@ describe('DatabaseController', function () {
           email_1: { email: 1 },
           _email_verify_token: { _email_verify_token: 1 },
           _perishable_token: { _perishable_token: 1 },
+          _auth_data_custom_id: { '_auth_data_custom.id': 1 },
+          _auth_data_facebook_id: { '_auth_data_facebook.id': 1 },
+          _auth_data_myoauth_id: { '_auth_data_myoauth.id': 1 },
+          _auth_data_shortLivedAuth_id: { '_auth_data_shortLivedAuth.id': 1 },
+          _auth_data_anonymous_id: { '_auth_data_anonymous.id': 1 },
         });
       }
     );
diff --git a/spec/ParseUser.spec.js b/spec/ParseUser.spec.js
index 038058905..ca0dded5e 100644
--- a/spec/ParseUser.spec.js
+++ b/spec/ParseUser.spec.js
@@ -337,7 +337,7 @@ describe('Parse.User testing', () => {
     expect(newUser).not.toBeUndefined();
   });
 
-  it('should be let masterKey lock user out with authData', async () => {
+  it_only_db('mongo')('should reject duplicate authData when masterKey locks user out (mongo)', async () => {
     const response = await request({
       method: 'POST',
       url: 'http://localhost:8378/1/classes/_User',
@@ -353,15 +353,13 @@ describe('Parse.User testing', () => {
     });
     const body = response.data;
     const objectId = body.objectId;
-    const sessionToken = body.sessionToken;
-    expect(sessionToken).toBeDefined();
+    expect(body.sessionToken).toBeDefined();
     expect(objectId).toBeDefined();
     const user = new Parse.User();
     user.id = objectId;
     const ACL = new Parse.ACL();
     user.setACL(ACL);
     await user.save(null, { useMasterKey: true });
-    // update the user
     const options = {
       method: 'POST',
       url: `http://localhost:8378/1/classes/_User/`,
@@ -377,8 +375,61 @@ describe('Parse.User testing', () => {
         },
       },
     };
-    const res = await request(options);
-    expect(res.data.objectId).not.toEqual(objectId);
+    try {
+      await request(options);
+      fail('should have thrown');
+    } catch (err) {
+      expect(err.data.code).toBe(208);
+      expect(err.data.error).toBe('this auth is already used');
+    }
+  });
+
+  it_only_db('postgres')('should reject duplicate authData when masterKey locks user out (postgres)', async () => {
+    await reconfigureServer();
+    const response = await request({
+      method: 'POST',
+      url: 'http://localhost:8378/1/classes/_User',
+      headers: {
+        'X-Parse-Application-Id': Parse.applicationId,
+        'X-Parse-REST-API-Key': 'rest',
+        'Content-Type': 'application/json',
+      },
+      body: {
+        key: 'value',
+        authData: { anonymous: { id: '00000000-0000-0000-0000-000000000001' } },
+      },
+    });
+    const body = response.data;
+    const objectId = body.objectId;
+    expect(body.sessionToken).toBeDefined();
+    expect(objectId).toBeDefined();
+    const user = new Parse.User();
+    user.id = objectId;
+    const ACL = new Parse.ACL();
+    user.setACL(ACL);
+    await user.save(null, { useMasterKey: true });
+    const options = {
+      method: 'POST',
+      url: `http://localhost:8378/1/classes/_User/`,
+      headers: {
+        'X-Parse-Application-Id': Parse.applicationId,
+        'X-Parse-REST-API-Key': 'rest',
+        'Content-Type': 'application/json',
+      },
+      body: {
+        key: 'otherValue',
+        authData: {
+          anonymous: { id: '00000000-0000-0000-0000-000000000001' },
+        },
+      },
+    };
+    try {
+      await request(options);
+      fail('should have thrown');
+    } catch (err) {
+      expect(err.data.code).toBe(208);
+      expect(err.data.error).toBe('this auth is already used');
+    }
   });
 
   it('user login with files', done => {
diff --git a/src/Adapters/Storage/Mongo/MongoStorageAdapter.js b/src/Adapters/Storage/Mongo/MongoStorageAdapter.js
index 50fd34886..a6ac012f3 100644
--- a/src/Adapters/Storage/Mongo/MongoStorageAdapter.js
+++ b/src/Adapters/Storage/Mongo/MongoStorageAdapter.js
@@ -530,6 +530,13 @@ export class MongoStorageAdapter implements StorageAdapter {
             if (matches && Array.isArray(matches)) {
               err.userInfo = { duplicated_field: matches[1] };
             }
+            // Check for authData unique index violations
+            if (!err.userInfo) {
+              const authDataMatch = error.message.match(/index:\s+(_auth_data_[a-zA-Z0-9_]+_id)/);
+              if (authDataMatch) {
+                err.userInfo = { duplicated_field: authDataMatch[1] };
+              }
+            }
           }
           throw err;
         }
@@ -605,10 +612,27 @@ export class MongoStorageAdapter implements StorageAdapter {
       .then(result => mongoObjectToParseObject(className, result, schema))
       .catch(error => {
         if (error.code === 11000) {
-          throw new Parse.Error(
+          logger.error('Duplicate key error:', error.message);
+          const err = new Parse.Error(
             Parse.Error.DUPLICATE_VALUE,
             'A duplicate value for a field with unique values was provided'
           );
+          err.underlyingError = error;
+          if (error.message) {
+            const matches = error.message.match(
+              /index:[\sa-zA-Z0-9_\-\.]+\$?([a-zA-Z_-]+)_1/
+            );
+            if (matches && Array.isArray(matches)) {
+              err.userInfo = { duplicated_field: matches[1] };
+            }
+            if (!err.userInfo) {
+              const authDataMatch = error.message.match(/index:\s+(_auth_data_[a-zA-Z0-9_]+_id)/);
+              if (authDataMatch) {
+                err.userInfo = { duplicated_field: authDataMatch[1] };
+              }
+            }
+          }
+          throw err;
         }
         throw error;
       })
@@ -764,6 +788,32 @@ export class MongoStorageAdapter implements StorageAdapter {
       .catch(err => this.handleError(err));
   }
 
+  // Creates a unique sparse index on _auth_data_<provider>.id to prevent
+  // race conditions during concurrent signups with the same authData.
+  ensureAuthDataUniqueness(provider: string) {
+    return this._adaptiveCollection('_User')
+      .then(collection =>
+        collection._mongoCollection.createIndex(
+          { [`_auth_data_${provider}.id`]: 1 },
+          { unique: true, sparse: true, background: true, name: `_auth_data_${provider}_id` }
+        )
+      )
+      .catch(error => {
+        if (error.code === 11000) {
+          throw new Parse.Error(
+            Parse.Error.DUPLICATE_VALUE,
+            'Tried to ensure field uniqueness for a class that already has duplicates.'
+          );
+        }
+        // Ignore "index already exists with same name" or "index already exists with different options"
+        if (error.code === 85 || error.code === 86) {
+          return;
+        }
+        throw error;
+      })
+      .catch(err => this.handleError(err));
+  }
+
   // Used in tests
   _rawFind(className: string, query: QueryType) {
     return this._adaptiveCollection(className)
diff --git a/src/Adapters/Storage/Postgres/PostgresStorageAdapter.js b/src/Adapters/Storage/Postgres/PostgresStorageAdapter.js
index 7eaafcbde..6adf0b170 100644
--- a/src/Adapters/Storage/Postgres/PostgresStorageAdapter.js
+++ b/src/Adapters/Storage/Postgres/PostgresStorageAdapter.js
@@ -1479,9 +1479,15 @@ export class PostgresStorageAdapter implements StorageAdapter {
           );
           err.underlyingError = error;
           if (error.constraint) {
-            const matches = error.constraint.match(/unique_([a-zA-Z]+)/);
-            if (matches && Array.isArray(matches)) {
-              err.userInfo = { duplicated_field: matches[1] };
+            // Check for authData unique index violations first
+            const authDataMatch = error.constraint.match(/_User_unique_authData_([a-zA-Z0-9_]+)_id/);
+            if (authDataMatch) {
+              err.userInfo = { duplicated_field: `_auth_data_${authDataMatch[1]}` };
+            } else {
+              const matches = error.constraint.match(/unique_([a-zA-Z]+)/);
+              if (matches && Array.isArray(matches)) {
+                err.userInfo = { duplicated_field: matches[1] };
+              }
             }
           }
           error = err;
@@ -1801,7 +1807,30 @@ export class PostgresStorageAdapter implements StorageAdapter {
 
     const whereClause = where.pattern.length > 0 ? `WHERE ${where.pattern}` : '';
     const qs = `UPDATE $1:name SET ${updatePatterns.join()} ${whereClause} RETURNING *`;
-    const promise = (transactionalSession ? transactionalSession.t : this._client).any(qs, values);
+    const promise = (transactionalSession ? transactionalSession.t : this._client)
+      .any(qs, values)
+      .catch(error => {
+        if (error.code === PostgresUniqueIndexViolationError) {
+          const err = new Parse.Error(
+            Parse.Error.DUPLICATE_VALUE,
+            'A duplicate value for a field with unique values was provided'
+          );
+          err.underlyingError = error;
+          if (error.constraint) {
+            const authDataMatch = error.constraint.match(/_User_unique_authData_([a-zA-Z0-9_]+)_id/);
+            if (authDataMatch) {
+              err.userInfo = { duplicated_field: `_auth_data_${authDataMatch[1]}` };
+            } else {
+              const matches = error.constraint.match(/unique_([a-zA-Z]+)/);
+              if (matches && Array.isArray(matches)) {
+                err.userInfo = { duplicated_field: matches[1] };
+              }
+            }
+          }
+          throw err;
+        }
+        throw error;
+      });
     if (transactionalSession) {
       transactionalSession.batch.push(promise);
     }
@@ -2044,6 +2073,31 @@ export class PostgresStorageAdapter implements StorageAdapter {
     });
   }
 
+  // Creates a unique index on authData-><provider>->>'id' to prevent
+  // race conditions during concurrent signups with the same authData.
+  async ensureAuthDataUniqueness(provider: string) {
+    const indexName = `_User_unique_authData_${provider}_id`;
+    const qs = `CREATE UNIQUE INDEX IF NOT EXISTS $1:name ON "_User" (("authData"->$2::text->>'id')) WHERE "authData"->$2::text->>'id' IS NOT NULL`;
+    await this._client.none(qs, [indexName, provider]).catch(error => {
+      if (
+        error.code === PostgresDuplicateRelationError &&
+        error.message.includes(indexName)
+      ) {
+        // Index already exists. Ignore error.
+      } else if (
+        error.code === PostgresUniqueIndexViolationError &&
+        error.message.includes(indexName)
+      ) {
+        throw new Parse.Error(
+          Parse.Error.DUPLICATE_VALUE,
+          'Tried to ensure field uniqueness for a class that already has duplicates.'
+        );
+      } else {
+        throw error;
+      }
+    });
+  }
+
   // Executes a count.
   async count(
     className: string,
diff --git a/src/Controllers/DatabaseController.js b/src/Controllers/DatabaseController.js
index 0f154693b..a7ed99aab 100644
--- a/src/Controllers/DatabaseController.js
+++ b/src/Controllers/DatabaseController.js
@@ -1838,6 +1838,30 @@ class DatabaseController {
           throw error;
         });
     }
+    // Create unique indexes for authData providers to prevent race conditions
+    // during concurrent signups with the same authData
+    if (
+      databaseOptions.createIndexAuthDataUniqueness !== false &&
+      typeof this.adapter.ensureAuthDataUniqueness === 'function'
+    ) {
+      const authProviders = Object.keys(this.options.auth || {});
+      if (this.options.enableAnonymousUsers !== false) {
+        if (!authProviders.includes('anonymous')) {
+          authProviders.push('anonymous');
+        }
+      }
+      await Promise.all(
+        authProviders.map(provider =>
+          this.adapter.ensureAuthDataUniqueness(provider).catch(error => {
+            logger.warn(
+              `Unable to ensure uniqueness for auth data provider "${provider}": `,
+              error
+            );
+          })
+        )
+      );
+    }
+
     await this.adapter.updateSchemaWithIndexes();
   }
 
diff --git a/src/Options/Definitions.js b/src/Options/Definitions.js
index 6a9ca20df..03db895da 100644
--- a/src/Options/Definitions.js
+++ b/src/Options/Definitions.js
@@ -110,7 +110,7 @@ module.exports.ParseServerOptions = {
   auth: {
     env: 'PARSE_SERVER_AUTH_PROVIDERS',
     help:
-      'Configuration for your authentication providers, as stringified JSON. See http://docs.parseplatform.org/parse-server/guide/#oauth-and-3rd-party-authentication',
+      "Configuration for your authentication providers, as stringified JSON. See http://docs.parseplatform.org/parse-server/guide/#oauth-and-3rd-party-authentication<br><br>Provider names must start with a letter and contain only letters, digits, and underscores (`/^[A-Za-z][A-Za-z0-9_]*$/`). This is because each provider name is used to construct a database field (`_auth_data_<provider>`), which must comply with Parse Server's field naming rules.",
     action: parsers.objectParser,
   },
   cacheAdapter: {
@@ -1248,6 +1248,13 @@ module.exports.DatabaseOptions = {
       'The MongoDB driver option to specify the amount of time, in milliseconds, to wait to establish a single TCP socket connection to the server before raising an error. Specifying 0 disables the connection timeout.',
     action: parsers.numberParser('connectTimeoutMS'),
   },
+  createIndexAuthDataUniqueness: {
+    env: 'PARSE_SERVER_DATABASE_CREATE_INDEX_AUTH_DATA_UNIQUENESS',
+    help:
+      'Set to `true` to automatically create unique indexes on the authData fields of the _User collection for each configured auth provider on server start, including `anonymous` when anonymous users are enabled. These indexes prevent race conditions during concurrent signups with the same authData. Set to `false` to skip index creation. Default is `true`.<br><br>\u26A0\uFE0F When setting this option to `false` to manually create the indexes, keep in mind that the otherwise automatically created indexes may change in the future to be optimized for the internal usage by Parse Server.',
+    action: parsers.booleanParser,
+    default: true,
+  },
   createIndexRoleName: {
     env: 'PARSE_SERVER_DATABASE_CREATE_INDEX_ROLE_NAME',
     help:
diff --git a/src/Options/docs.js b/src/Options/docs.js
index 89922709f..7df315850 100644
--- a/src/Options/docs.js
+++ b/src/Options/docs.js
@@ -21,7 +21,7 @@
  * @property {Adapter<AnalyticsAdapter>} analyticsAdapter Adapter module for the analytics
  * @property {String} appId Your Parse Application ID
  * @property {String} appName Sets the app name
- * @property {Object} auth Configuration for your authentication providers, as stringified JSON. See http://docs.parseplatform.org/parse-server/guide/#oauth-and-3rd-party-authentication
+ * @property {Object} auth Configuration for your authentication providers, as stringified JSON. See http://docs.parseplatform.org/parse-server/guide/#oauth-and-3rd-party-authentication<br><br>Provider names must start with a letter and contain only letters, digits, and underscores (`/^[A-Za-z][A-Za-z0-9_]*$/`). This is because each provider name is used to construct a database field (`_auth_data_<provider>`), which must comply with Parse Server's field naming rules.
  * @property {Adapter<CacheAdapter>} cacheAdapter Adapter module for the cache
  * @property {Number} cacheMaxSize Sets the maximum size for the in memory cache, defaults to 10000
  * @property {Number} cacheTTL Sets the TTL for the in memory cache (in ms), defaults to 5000 (5 seconds)
@@ -278,6 +278,7 @@
  * @property {Number} autoSelectFamilyAttemptTimeout The MongoDB driver option to specify the amount of time in milliseconds to wait for a connection attempt to finish before trying the next address when using the autoSelectFamily option. If set to a positive integer less than 10, the value 10 is used instead.
  * @property {Union} compressors The MongoDB driver option to specify an array or comma-delimited string of compressors to enable network compression for communication between this client and a mongod/mongos instance.
  * @property {Number} connectTimeoutMS The MongoDB driver option to specify the amount of time, in milliseconds, to wait to establish a single TCP socket connection to the server before raising an error. Specifying 0 disables the connection timeout.
+ * @property {Boolean} createIndexAuthDataUniqueness Set to `true` to automatically create unique indexes on the authData fields of the _User collection for each configured auth provider on server start, including `anonymous` when anonymous users are enabled. These indexes prevent race conditions during concurrent signups with the same authData. Set to `false` to skip index creation. Default is `true`.<br><br>⚠️ When setting this option to `false` to manually create the indexes, keep in mind that the otherwise automatically created indexes may change in the future to be optimized for the internal usage by Parse Server.
  * @property {Boolean} createIndexRoleName Set to `true` to automatically create a unique index on the name field of the _Role collection on server start. Set to `false` to skip index creation. Default is `true`.<br><br>⚠️ When setting this option to `false` to manually create the index, keep in mind that the otherwise automatically created index may change in the future to be optimized for the internal usage by Parse Server.
  * @property {Boolean} createIndexUserEmail Set to `true` to automatically create indexes on the email field of the _User collection on server start. Set to `false` to skip index creation. Default is `true`.<br><br>⚠️ When setting this option to `false` to manually create the index, keep in mind that the otherwise automatically created index may change in the future to be optimized for the internal usage by Parse Server.
  * @property {Boolean} createIndexUserEmailCaseInsensitive Set to `true` to automatically create a case-insensitive index on the email field of the _User collection on server start. Set to `false` to skip index creation. Default is `true`.<br><br>⚠️ When setting this option to `false` to manually create the index, keep in mind that the otherwise automatically created index may change in the future to be optimized for the internal usage by Parse Server.
diff --git a/src/Options/index.js b/src/Options/index.js
index d36aafb4b..7de4eb768 100644
--- a/src/Options/index.js
+++ b/src/Options/index.js
@@ -164,7 +164,7 @@ export interface ParseServerOptions {
   :ENV: PARSE_SERVER_ALLOW_CUSTOM_OBJECT_ID
   :DEFAULT: false */
   allowCustomObjectId: ?boolean;
-  /* Configuration for your authentication providers, as stringified JSON. See http://docs.parseplatform.org/parse-server/guide/#oauth-and-3rd-party-authentication
+  /* Configuration for your authentication providers, as stringified JSON. See http://docs.parseplatform.org/parse-server/guide/#oauth-and-3rd-party-authentication<br><br>Provider names must start with a letter and contain only letters, digits, and underscores (`/^[A-Za-z][A-Za-z0-9_]*$/`). This is because each provider name is used to construct a database field (`_auth_data_<provider>`), which must comply with Parse Server's field naming rules.
   :ENV: PARSE_SERVER_AUTH_PROVIDERS */
   auth: ?{ [string]: AuthAdapter };
   /* Enable (or disable) insecure auth adapters, defaults to true. Insecure auth adapters are deprecated and it is recommended to disable them.
@@ -775,6 +775,9 @@ export interface DatabaseOptions {
   /* Set to `true` to automatically create a case-insensitive index on the username field of the _User collection on server start. Set to `false` to skip index creation. Default is `true`.<br><br>⚠️ When setting this option to `false` to manually create the index, keep in mind that the otherwise automatically created index may change in the future to be optimized for the internal usage by Parse Server.
   :DEFAULT: true */
   createIndexUserUsernameCaseInsensitive: ?boolean;
+  /* Set to `true` to automatically create unique indexes on the authData fields of the _User collection for each configured auth provider on server start, including `anonymous` when anonymous users are enabled. These indexes prevent race conditions during concurrent signups with the same authData. Set to `false` to skip index creation. Default is `true`.<br><br>⚠️ When setting this option to `false` to manually create the indexes, keep in mind that the otherwise automatically created indexes may change in the future to be optimized for the internal usage by Parse Server.
+  :DEFAULT: true */
+  createIndexAuthDataUniqueness: ?boolean;
   /* Set to `true` to automatically create a unique index on the name field of the _Role collection on server start. Set to `false` to skip index creation. Default is `true`.<br><br>⚠️ When setting this option to `false` to manually create the index, keep in mind that the otherwise automatically created index may change in the future to be optimized for the internal usage by Parse Server.
   :DEFAULT: true */
   createIndexRoleName: ?boolean;
diff --git a/src/RestWrite.js b/src/RestWrite.js
index a0de5577a..0f5f8ec61 100644
--- a/src/RestWrite.js
+++ b/src/RestWrite.js
@@ -514,6 +514,16 @@ RestWrite.prototype.getUserId = function () {
   }
 };
 
+RestWrite.prototype._throwIfAuthDataDuplicate = function (error) {
+  if (
+    this.className === '_User' &&
+    error?.code === Parse.Error.DUPLICATE_VALUE &&
+    error.userInfo?.duplicated_field?.startsWith('_auth_data_')
+  ) {
+    throw new Parse.Error(Parse.Error.ACCOUNT_ALREADY_LINKED, 'this auth is already used');
+  }
+};
+
 // Developers are allowed to change authData via before save trigger
 // we need after before save to ensure that the developer
 // is not currently duplicating auth data ID
@@ -644,12 +654,17 @@ RestWrite.prototype.handleAuthData = async function (authData) {
         // uses the `doNotSave` option. Just update the authData part
         // Then we're good for the user, early exit of sorts
         if (Object.keys(this.data.authData).length) {
-          await this.config.database.update(
-            this.className,
-            { objectId: this.data.objectId },
-            { authData: this.data.authData },
-            {}
-          );
+          try {
+            await this.config.database.update(
+              this.className,
+              { objectId: this.data.objectId },
+              { authData: this.data.authData },
+              {}
+            );
+          } catch (error) {
+            this._throwIfAuthDataDuplicate(error);
+            throw error;
+          }
         }
       }
     }
@@ -1545,6 +1560,10 @@ RestWrite.prototype.runDatabaseOperation = function () {
           false,
           this.validSchemaController
         )
+        .catch(error => {
+          this._throwIfAuthDataDuplicate(error);
+          throw error;
+        })
         .then(response => {
           response.updatedAt = this.updatedAt;
           this._updateResponseWithData(response, this.data);
@@ -1579,6 +1598,8 @@ RestWrite.prototype.runDatabaseOperation = function () {
           throw error;
         }
 
+        this._throwIfAuthDataDuplicate(error);
+
         // Quick check, if we were able to infer the duplicated field name
         if (error && error.userInfo && error.userInfo.duplicated_field === 'username') {
           throw new Parse.Error(
diff --git a/src/defaults.js b/src/defaults.js
index 07eeb5136..4f0e62611 100644
--- a/src/defaults.js
+++ b/src/defaults.js
@@ -38,6 +38,7 @@ export const DefaultMongoURI = DefinitionDefaults.databaseURI;
 // before passing to MongoDB client
 export const ParseServerDatabaseOptions = [
   'allowPublicExplain',
+  'createIndexAuthDataUniqueness',
   'createIndexRoleName',
   'createIndexUserEmail',
   'createIndexUserEmailCaseInsensitive',