ignore duplicate assemblies

src/EditorFeatures/CSharpTest/Completion/CompletionServiceTests.cs

@@ -14,15 +14,7 @@ public class CompletionServiceTests
         [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
         public void AcquireCompletionService()
         {
-            var hostServices = MefHostServices.Create(
-                MefHostServices.DefaultAssemblies.Concat(
-                    new[]
-                    {
-                        typeof(CompletionService).Assembly,
-                        typeof(CSharpCompletionService).Assembly
-                    }));
-
-            var workspace = new AdhocWorkspace(hostServices);
+            var workspace = new AdhocWorkspace();
 
             var document = workspace
                 .AddProject("TestProject", LanguageNames.CSharp)

src/EditorFeatures/VisualBasicTest/Completion/CompletionServiceTests.vb

@@ -8,12 +8,7 @@ Namespace Microsoft.CodeAnalysis.Editor.VisualBasic.UnitTests.Completion
     Public Class CompletionServiceTests
         <Fact, Trait(Traits.Feature, Traits.Features.Completion)>
         Public Sub AcquireCompletionService()
-            Dim hostServices = MefHostServices.Create(
-                MefHostServices.DefaultAssemblies.Concat({
-                    GetType(CompletionService).Assembly,
-                    GetType(VisualBasicCompletionService).Assembly}))
-
-            Dim workspace = New AdhocWorkspace(hostServices)
+            Dim workspace = New AdhocWorkspace()
 
             Dim document = workspace _
                 .AddProject("TestProject", LanguageNames.VisualBasic) _

src/Workspaces/Core/Portable/Workspace/Host/Mef/MefHostServices.cs

@@ -37,7 +37,7 @@ public static MefHostServices Create(IEnumerable<System.Reflection.Assembly> ass
                 throw new ArgumentNullException(nameof(assemblies));
             }
 
-            var compositionConfiguration = new ContainerConfiguration().WithAssemblies(assemblies);
+            var compositionConfiguration = new ContainerConfiguration().WithAssemblies(assemblies.Distinct());
             var container = compositionConfiguration.CreateContainer();
             return new MefHostServices(container);
         }