Ensure that MSBuildWorkspace deals with duplicate ProjectReferences

The workspace isn't supposed to allow duplicate ProjectReferences, but
currently due to https://github.com/dotnet/roslyn/issues/12101 it does.
This dedups (correctly) at the surface and also adds a test to ensure
this isn't broken.

Fixes https://github.com/dotnet/roslyn/issues/31390.

src/Workspaces/Core/MSBuild/MSBuild/MSBuildProjectLoader.Worker_ResolveReferences.cs

@@ -17,10 +17,10 @@ private partial class Worker
         {
             private readonly struct ResolvedReferences
             {
-                public ImmutableArray<ProjectReference> ProjectReferences { get; }
+                public ImmutableHashSet<ProjectReference> ProjectReferences { get; }
                 public ImmutableArray<MetadataReference> MetadataReferences { get; }
 
-                public ResolvedReferences(ImmutableArray<ProjectReference> projectReferences, ImmutableArray<MetadataReference> metadataReferences)
+                public ResolvedReferences(ImmutableHashSet<ProjectReference> projectReferences, ImmutableArray<MetadataReference> metadataReferences)
                 {
                     ProjectReferences = projectReferences;
                     MetadataReferences = metadataReferences;
@@ -50,14 +50,14 @@ private class ResolvedReferencesBuilder
                 /// </summary>
                 private readonly HashSet<int> _indicesToRemove;
 
-                private readonly ImmutableArray<ProjectReference>.Builder _projectReferences;
+                private readonly ImmutableHashSet<ProjectReference>.Builder _projectReferences;
 
                 public ResolvedReferencesBuilder(IEnumerable<MetadataReference> metadataReferences)
                 {
                     _metadataReferences = metadataReferences.ToImmutableArray();
                     _pathToIndicesMap = CreatePathToIndexMap(_metadataReferences);
                     _indicesToRemove = new HashSet<int>();
-                    _projectReferences = ImmutableArray.CreateBuilder<ProjectReference>();
+                    _projectReferences = ImmutableHashSet.CreateBuilder<ProjectReference>();
                 }
 
                 private static ImmutableDictionary<string, HashSet<int>> CreatePathToIndexMap(ImmutableArray<MetadataReference> metadataReferences)
@@ -166,7 +166,7 @@ private ImmutableArray<MetadataReference> GetMetadataReferences()
                     return builder.ToImmutable();
                 }
 
-                private ImmutableArray<ProjectReference> GetProjectReferences()
+                private ImmutableHashSet<ProjectReference> GetProjectReferences()
                     => _projectReferences.ToImmutable();
 
                 public ResolvedReferences ToResolvedReferences()

src/Workspaces/MSBuildTest/MSBuildWorkspaceTests.cs

@@ -3606,6 +3606,28 @@ public async Task TestOpenProjectAsync_MalformedAdditionalFilePath()
             }
         }
 
+        [ConditionalFact(typeof(VisualStudioMSBuildInstalled)), Trait(Traits.Feature, Traits.Features.MSBuildWorkspace)]
+        [WorkItem(31390, "https://github.com/dotnet/roslyn/issues/31390")]
+        public async Task TestDuplicateProjectReference()
+        {
+            var files = GetDuplicateProjectReferenceSolutionFiles();
+            CreateFiles(files);
+
+            var fullPath = GetSolutionFileName(@"CSharpProjectReference.sln");
+
+            using (var workspace = CreateMSBuildWorkspace())
+            {
+                var solution = await workspace.OpenSolutionAsync(fullPath);
+                var project = solution.Projects.Single(p => p.FilePath.EndsWith("CSharpProject_ProjectReference.csproj"));
+
+                Assert.Single(project.ProjectReferences);
+
+                var compilation = await project.GetCompilationAsync();
+
+                Assert.Single(compilation.References.OfType<CompilationReference>());
+            }
+        }
+
         private class InMemoryAssemblyLoader : IAnalyzerAssemblyLoader
         {
             public void AddDependencyLocation(string fullPath)

src/Workspaces/MSBuildTest/Resources/ProjectFiles/CSharp/DuplicateProjectReference.csproj

+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c)  Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information. -->
+<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition="'$(Configuration)' == ''">Debug</Configuration>
+    <Platform Condition="'$(Platform)' == ''">AnyCPU</Platform>
+    <PlatformTarget>AnyCPU</PlatformTarget>
+    <ProjectGuid>{3DA47258-9CAB-4FB9-9C04-1F367A2C2700}</ProjectGuid>
+    <OutputType>Exe</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>CSharpProject_ProjectToProjectReference</RootNamespace>
+    <AssemblyName>CSharpProject_ProjectToProjectReference</AssemblyName>
+    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|AnyCPU'">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|AnyCPU'">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="Microsoft.CSharp" />
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+    <Reference Include="System.Data" />
+    <Reference Include="System.Xml" />
+    <Reference Include="System.Xml.Linq" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="CSharpConsole.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="CSharpProject.csproj">
+      <Project>{686dd036-86aa-443e-8a10-ddb43266a8c4}</Project>
+      <Name>CSharpProject</Name>
+    </ProjectReference>
+    <ProjectReference Include="CSharpProject.csproj">
+      <Project>{686dd036-86aa-443e-8a10-ddb43266a8c4}</Project>
+      <Name>CSharpProject</Name>
+    </ProjectReference>
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it.
+       Other similar extension points exist, see Microsoft.Common.targets.
+  <Target Name="BeforeBuild">
+  </Target>
+  <Target Name="AfterBuild">
+  </Target>
+  -->
+</Project>
\ No newline at end of file

src/Workspaces/MSBuildTest/TestFiles/Resources.cs

@@ -115,6 +115,7 @@ public static class CSharp
                 public static string CircularProjectReferences_CircularCSharpProject2 => GetText("CircularProjectReferences.CircularCSharpProject2.csproj");
                 public static string CSharpProject => GetText("ProjectFiles.CSharp.CSharpProject.csproj");
                 public static string DuplicateFile => GetText("ProjectFiles.CSharp.DuplicateFile.csproj");
+                public static string DuplicateProjectReference => GetText("ProjectFiles.CSharp.DuplicateProjectReference.csproj");
                 public static string DuplicatedGuidLibrary1 => GetText("ProjectFiles.CSharp.DuplicatedGuidLibrary1.csproj");
                 public static string DuplicatedGuidLibrary2 => GetText("ProjectFiles.CSharp.DuplicatedGuidLibrary2.csproj");
                 public static string DuplicatedGuidLibrary3 => GetText("ProjectFiles.CSharp.DuplicatedGuidLibrary3.csproj");

src/Workspaces/MSBuildTest/WorkspaceTestBase.cs

@@ -225,6 +225,20 @@ protected FileSet GetProjectReferenceSolutionFiles()
                 (@"CSharpProject\CSharpConsole.cs", Resources.SourceFiles.CSharp.CSharpConsole));
         }
 
+        protected FileSet GetDuplicateProjectReferenceSolutionFiles()
+        {
+            return new FileSet(
+                (@"NuGet.Config", Resources.NuGet_Config),
+                (@"Directory.Build.props", Resources.Directory_Build_props),
+                (@"Directory.Build.targets", Resources.Directory_Build_targets),
+                (@"CSharpProjectReference.sln", Resources.SolutionFiles.CSharp_ProjectReference),
+                (@"CSharpProject\CSharpProject.csproj", Resources.ProjectFiles.CSharp.CSharpProject),
+                (@"CSharpProject\CSharpClass.cs", Resources.SourceFiles.CSharp.CSharpClass),
+                (@"CSharpProject\Properties\AssemblyInfo.cs", Resources.SourceFiles.CSharp.AssemblyInfo),
+                (@"CSharpProject\CSharpProject_ProjectReference.csproj", Resources.ProjectFiles.CSharp.DuplicateProjectReference),
+                (@"CSharpProject\CSharpConsole.cs", Resources.SourceFiles.CSharp.CSharpConsole));
+        }
+
         protected FileSet GetAnalyzerReferenceSolutionFiles()
         {
             return new FileSet(