SDK does not correctly resolve RazorExtension when multiple instances of RazorConfiguration are present

Fixes https://github.com/aspnet/Razor/issues/2721

src/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Sdk.Razor.CurrentVersion.targets

@@ -276,10 +276,10 @@ Copyright (c) .NET Foundation. All rights reserved.
     Condition="'$(RazorDefaultConfiguration)'!=''">
 
     <ItemGroup>
-      <ResolvedRazorConfiguration Include="@(RazorConfiguration)" Condition="'%(RazorConfiguration.Identity)'=='$(RazorDefaultConfiguration)'" />
+      <ResolvedRazorConfiguration Include="@(RazorConfiguration->WithMetadataValue('Identity', '$(RazorDefaultConfiguration)')->Distinct())" />
     </ItemGroup>
 
-    <FindInList List="@(RazorExtension)" ItemSpecToFind="@(RazorConfiguration->Metadata('Extensions'))">
+    <FindInList List="@(RazorExtension)" ItemSpecToFind="@(ResolvedRazorConfiguration->Metadata('Extensions'))">
       <Output TaskParameter="ItemFound" ItemName="ResolvedRazorExtension" />
     </FindInList>
   </Target>

test/Microsoft.NET.Sdk.Razor.Test/IntegrationTests/BuildIntegrationTest.cs

@@ -630,6 +630,28 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
             Assert.FileExists(result, OutputPath, "SimpleMvc21.Views.pdb");
         }
 
+        [Fact]
+        [InitializeTestProject("SimpleMvc21")]
+        public async Task Building_WorksWhenMultipleRazorConfigurationsArePresent()
+        {
+            TargetFramework = "netcoreapp2.1";
+            AddProjectFileContent(@"
+<ItemGroup>
+    <RazorConfiguration Include=""MVC-2.1"">
+      <Extensions>MVC-2.1;$(CustomRazorExtension)</Extensions>
+    </RazorConfiguration>
+</ItemGroup>");
+
+            // Build
+            var result = await DotnetMSBuild("Build");
+
+            Assert.BuildPassed(result);
+            Assert.FileExists(result, OutputPath, "SimpleMvc21.dll");
+            Assert.FileExists(result, OutputPath, "SimpleMvc21.pdb");
+            Assert.FileExists(result, OutputPath, "SimpleMvc21.Views.dll");
+            Assert.FileExists(result, OutputPath, "SimpleMvc21.Views.pdb");
+        }
+
         private static DependencyContext ReadDependencyContext(string depsFilePath)
         {
             var reader = new DependencyContextJsonReader();

test/testapps/SimpleMvc21/SimpleMvc21.csproj

@@ -9,6 +9,8 @@
     <TargetFramework>netcoreapp2.1</TargetFramework>
   </PropertyGroup>
 
+  <!-- Test Placeholder -->
+
   <PropertyGroup Condition="'$(RunningAsTest)' == ''">
     <!-- We don't want to run build server when not running as tests. -->
     <UseRazorBuildServer>false</UseRazorBuildServer>

test/testapps/SimpleMvc21/Views/_ViewImports.cshtml

@@ -1,3 +1,4 @@
 @using SimpleMvc
 @using SimpleMvc.Models
 @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+@namespace SimpleMvc21