Deduplicate libraries

TestAssets/TestProjects/DuplicatedReferenceAssembly/TestApp/.noautobuild

@@ -0,0 +1 @@
+noautobuild

TestAssets/TestProjects/DuplicatedReferenceAssembly/TestApp/Program.cs

@@ -0,0 +1,10 @@
+namespace TestApp
+{
+    public class Program
+    {
+        public static int Main(string[] args)
+        {
+            return 0;
+        }
+    }
+}

TestAssets/TestProjects/DuplicatedReferenceAssembly/TestApp/project.json

@@ -0,0 +1,11 @@
+{
+  "version": "1.0.0",
+  "dependencies": {
+    "TestLibrary": "1.0.0",
+    "System.IO.Compression": "4.1.0-rc2-24027"
+  },
+  "frameworks": {
+    "net461": {
+    }
+  }
+}

TestAssets/TestProjects/DuplicatedReferenceAssembly/TestLibrary/.noautobuild

@@ -0,0 +1 @@
+noautobuild

TestAssets/TestProjects/DuplicatedReferenceAssembly/TestLibrary/Program.cs

@@ -0,0 +1,10 @@
+namespace TestLibrary
+{
+    public class Program
+    {
+        public static int Main(string[] args)
+        {
+            return 0;
+        }
+    }
+}

TestAssets/TestProjects/DuplicatedReferenceAssembly/TestLibrary/project.json

@@ -0,0 +1,10 @@
+{
+  "version": "1.0.0",
+  "frameworks": {
+    "net461": {
+      "frameworkAssemblies": {
+        "System.IO.Compression": ""
+      }
+    }
+  }
+}

TestAssets/TestProjects/DuplicatedReferenceAssembly/global.json

@@ -0,0 +1 @@
+{ }

src/Microsoft.DotNet.ProjectModel/ProjectContextBuilder.cs

@@ -497,6 +497,22 @@ namespace Microsoft.DotNet.ProjectModel
                     dependencyDescription.Parents.Add(library);
                 }
             }
+
+            // Deduplicate libraries with the same name
+            // Priority list is backwards so not found -1 would be last when sorting by descending
+            var priorities = new[] { LibraryType.Package, LibraryType.Project, LibraryType.ReferenceAssembly };
+            var nameGroups = libraries.Keys.ToLookup(libraryKey => libraryKey.Name);
+            foreach (var nameGroup in nameGroups)
+            {
+                var librariesToRemove = nameGroup
+                    .OrderByDescending(libraryKey => Array.IndexOf(priorities, libraryKey.LibraryType))
+                    .Skip(1);
+
+                foreach (var library in librariesToRemove)
+                {
+                    libraries.Remove(library);
+                }
+            }
         }
 
         private void ScanLibraries(LockFileTarget target,

test/Microsoft.DotNet.ProjectModel.Tests/PackageDependencyProviderTests.cs

@@ -6,6 +6,7 @@ using Microsoft.DotNet.Tools.Test.Utilities;
 using NuGet.Frameworks;
 using NuGet.Versioning;
 using Xunit;
+using System.IO;
 
 namespace Microsoft.DotNet.ProjectModel.Tests
 {
@@ -140,5 +141,18 @@ namespace Microsoft.DotNet.ProjectModel.Tests
             var diagnostics = context.LibraryManager.GetAllDiagnostics();
             Assert.False(diagnostics.Any(d => d.ErrorCode == ErrorCodes.DOTNET1011));
         }
+
+        [Fact]
+        public void NoDuplicatesWithProjectAndReferenceAssemblyWithSameName()
+        {
+            var instance = TestAssetsManager.CreateTestInstance("DuplicatedReferenceAssembly")
+                                            .WithLockFiles();
+            var context = new ProjectContextBuilder().WithProjectDirectory(Path.Combine(instance.TestRoot, "TestApp"))
+                                                     .WithTargetFramework("net461")
+                                                     .Build();
+
+            // Will fail with dupes if any
+            context.LibraryManager.GetLibraries().ToDictionary(l => l.Identity.Name);
+        }
     }
 }