Fix error in windows provider when the environment as an existing set of variables name that only differs by case (#6320)

- make the provider storage for the environment on windows ignore duplicates
- add tests to verify existing environment get-item behavior

src/System.Management.Automation/namespaces/EnvironmentProvider.cs

@@ -198,7 +198,15 @@ namespace Microsoft.PowerShell.Commands
             IDictionary environmentTable = Environment.GetEnvironmentVariables();
             foreach (DictionaryEntry entry in environmentTable)
             {
-                providerTable.Add((string)entry.Key, entry);
+                // Windows only: duplicate key (variable name that differs only in case)
+                // NOTE: Even though this shouldn't happen, it can, e.g. when npm
+                //       creates duplicate environment variables that differ only in case -
+                //       see https://github.com/PowerShell/PowerShell/issues/6305.
+                //       However, because retrieval *by name* later is invariably
+                //       case-Insensitive, in effect only a *single* variable exists.
+                //       We simply ask Environment.GetEnvironmentVariable() which value is
+                //       the effective one, and use that.
+                providerTable.TryAdd((string)entry.Key, entry);
             }
 
             return providerTable;

test/powershell/Modules/Microsoft.PowerShell.Management/Get-Item.Tests.ps1

@@ -116,4 +116,30 @@ Describe "Get-Item" -Tags "CI" {
             ${result} | Should -BeOfType "Microsoft.Win32.RegistryKey"
         }
     }
+
+    Context "Environment provider" -tag "CI" {
+        BeforeAll {
+            $env:testvar="b"
+            $env:testVar="a"
+        }
+
+        AfterAll {
+            Clear-Item -Path env:testvar -ErrorAction SilentlyContinue
+            Clear-Item -Path env:testVar -ErrorAction SilentlyContinue
+        }
+
+        It "get-item testVar" {
+            (get-item env:\testVar).Value | Should -BeExactly "a"
+        }
+
+        It "get-item is case-sensitive/insensitive as appropriate" {
+            $expectedValue = "b"
+            if($IsWindows)
+            {
+                $expectedValue = "a"
+            }
+
+            (get-item env:\testvar).Value | Should -BeExactly $expectedValue
+        }
+    }
 }