diff --git a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/AddType.cs b/src/Microsoft.PowerShell.Commands.Utility/commands/utility/AddType.cs
index bbd536aada..4e6fc67643 100644
--- a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/AddType.cs
+++ b/src/Microsoft.PowerShell.Commands.Utility/commands/utility/AddType.cs
@@ -616,7 +616,7 @@ namespace Microsoft.PowerShell.Commands
         // These dictionaries prevent reloading already loaded and unchanged code.
         // We don't worry about unbounded growing of the cache because in .Net Core 2.0 we can not unload assemblies.
         // TODO: review if we will be able to unload assemblies after migrating to .Net Core 2.1.
-        private static readonly Dictionary<string, int> s_sourceTypesCache = new Dictionary<string, int>();
+        private static readonly HashSet<string> s_sourceTypesCache = new HashSet<string>();
         private static readonly Dictionary<int, Assembly> s_sourceAssemblyCache = new Dictionary<int, Assembly>();
 
         private static readonly string s_defaultSdkDirectory = Utils.DefaultPowerShellAppBase;
@@ -1042,7 +1042,7 @@ namespace Microsoft.PowerShell.Commands
 
         private void CheckDuplicateTypes(Compilation compilation, out ConcurrentBag<string> newTypes)
         {
-            AllNamedTypeSymbolsVisitor visitor = new AllNamedTypeSymbolsVisitor(_syntaxTreesHash);
+            AllNamedTypeSymbolsVisitor visitor = new AllNamedTypeSymbolsVisitor();
             visitor.Visit(compilation.Assembly.GlobalNamespace);
 
             foreach (var symbolName in visitor.DuplicateSymbols)
@@ -1074,16 +1074,9 @@ namespace Microsoft.PowerShell.Commands
         // Visit symbols in all namespaces and collect duplicates.
         private class AllNamedTypeSymbolsVisitor : SymbolVisitor
         {
-            private readonly int _hash;
-
             public readonly ConcurrentBag<string> DuplicateSymbols = new ConcurrentBag<string>();
             public readonly ConcurrentBag<string> UniqueSymbols = new ConcurrentBag<string>();
 
-            public AllNamedTypeSymbolsVisitor(int hash)
-            {
-                _hash = hash;
-            }
-
             public override void VisitNamespace(INamespaceSymbol symbol)
             {
                 // Main cycle.
@@ -1099,12 +1092,9 @@ namespace Microsoft.PowerShell.Commands
                 // It is namespace-fully-qualified name
                 var symbolFullName = symbol.ToString();
 
-                if (s_sourceTypesCache.TryGetValue(symbolFullName, out int hash))
+                if (s_sourceTypesCache.TryGetValue(symbolFullName, out _))
                 {
-                    if (hash == _hash)
-                    {
-                        DuplicateSymbols.Add(symbolFullName);
-                    }
+                    DuplicateSymbols.Add(symbolFullName);
                 }
                 else
                 {
@@ -1117,7 +1107,7 @@ namespace Microsoft.PowerShell.Commands
         {
             foreach (var typeName in newTypes)
             {
-                s_sourceTypesCache.Add(typeName, _syntaxTreesHash);
+                s_sourceTypesCache.Add(typeName);
             }
         }
 
diff --git a/test/powershell/Modules/Microsoft.PowerShell.Utility/Add-Type.Tests.ps1 b/test/powershell/Modules/Microsoft.PowerShell.Utility/Add-Type.Tests.ps1
index 20ed4b0afa..6d0fadbc69 100644
--- a/test/powershell/Modules/Microsoft.PowerShell.Utility/Add-Type.Tests.ps1
+++ b/test/powershell/Modules/Microsoft.PowerShell.Utility/Add-Type.Tests.ps1
@@ -1,5 +1,6 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
+
 Describe "Add-Type" -Tags "CI" {
     BeforeAll {
         $guid = [Guid]::NewGuid().ToString().Replace("-","")
@@ -65,7 +66,7 @@ Describe "Add-Type" -Tags "CI" {
         $code = @"
 using System.Management.Automation;
 [System.Management.Automation.Cmdlet("Get", "Thing$guid", ConfirmImpact = System.Management.Automation.ConfirmImpact.High, SupportsPaging = true)]
-public class AttributeTest$guid : PSCmdlet
+public class SMAAttributeTest$guid : PSCmdlet
 {
     protected override void EndProcessing()
 
@@ -207,6 +208,15 @@ public class AttributeTest$guid : PSCmdlet
         { Add-Type -CompilerOptions "/platform:anycpuERROR" -Language CSharp -MemberDefinition "public static string TestString() { return ""}" -Name "TestType1" -Namespace "TestNS" -ErrorAction Stop } | Should -Throw -ErrorId "SOURCE_CODE_ERROR,Microsoft.PowerShell.Commands.AddTypeCommand"
     }
 
+    It "Throw if the type already exists" {
+        Add-Type -TypeDefinition "public class Foo$guid {}"
+
+        # The cmdlet writes TYPE_ALREADY_EXISTS for every duplicated type and then terminates with COMPILER_ERRORS.
+        # So here we check 2 errors.
+        { Add-Type -TypeDefinition "public class Foo$guid { public int Bar {get {return 42;} }" -ErrorAction SilentlyContinue } | Should -Throw -ErrorId "COMPILER_ERRORS,Microsoft.PowerShell.Commands.AddTypeCommand"
+        $error[1].FullyQualifiedErrorId | Should -BeExactly "TYPE_ALREADY_EXISTS,Microsoft.PowerShell.Commands.AddTypeCommand"
+    }
+
     It "OutputType parameter requires that the OutputAssembly parameter be specified." {
         $code = "public static string TestString() {}"
         { Add-Type -TypeDefinition $code -OutputType Library } | Should -Throw -ErrorId "OUTPUTTYPE_REQUIRES_ASSEMBLY,Microsoft.PowerShell.Commands.AddTypeCommand"