Get-help may return multiple instances of the same help file (#3410)

* Help was incorrectly returning multiple instances of the same help file if it existed under a culture path and the parent was in the search path as well

* updated test to use generated help file rather than an actual one

* Fixed test to have module in $pshome path and casing of en-US culture

src/System.Management.Automation/help/HelpProvider.cs

@@ -273,7 +273,11 @@ namespace System.Management.Automation
             Diagnostics.Assert(searchPaths != null,
                 "HelpSystem returned an null search path");
 
-            searchPaths.Add(GetDefaultShellSearchPath());
+            string defaultShellSearchPath = GetDefaultShellSearchPath();
+            if (!searchPaths.Contains(defaultShellSearchPath))
+            {
+                searchPaths.Add(defaultShellSearchPath);
+            }
 
             return searchPaths;
         }

src/System.Management.Automation/help/MUIFileSearcher.cs

@@ -161,7 +161,7 @@ namespace System.Management.Automation
                             string leafFileName = Path.GetFileName(file);
                             string uniqueToDirectory = Path.Combine(directory, leafFileName);
 
-                            if (!_uniqueMatches.Contains(uniqueToDirectory))
+                            if (!_result.Contains(path) && !_uniqueMatches.Contains(uniqueToDirectory))
                             {
                                 _result.Add(path);
                                 _uniqueMatches[uniqueToDirectory] = true;

test/powershell/engine/Help/HelpSystem.Tests.ps1

@@ -180,3 +180,24 @@ Describe "Validate that Get-Help returns provider-specific help" -Tags @('CI', '
         }
     }
 }
+
+Describe "Validate about_help.txt under culture specific folder works" -Tags @('CI') {
+    BeforeAll {
+        $modulePath = "$pshome\Modules\Test"
+        $null = New-Item -Path $modulePath\en-US -ItemType Directory -Force
+        New-ModuleManifest -Path $modulePath\test.psd1 -RootModule test.psm1
+        Set-Content -Path $modulePath\test.psm1 -Value "function foo{}"
+        Set-Content -Path $modulePath\en-US\about_testhelp.help.txt -Value "Hello" -NoNewline
+    }
+
+    AfterAll {
+        Remove-Item $modulePath -Recurse -Force
+    }
+
+    It "Get-Help should return help text and not multiple HelpInfo objects when help is under `$pshome path" {
+
+        $help = Get-Help about_testhelp
+        $help.count | Should Be 1
+        $help | Should BeExactly "Hello"
+    }
+}