Only remove duplicate files with the exact same soure path.

Throw exception for files with conflicting package path

Author: thorgeir

src/Build/NuGet.Build.Packaging.Tasks/CreatePackage.cs

@@ -148,8 +148,18 @@ void AddFiles(Manifest manifest)
 			// Remove duplicate files
 			var contents = Contents
 				.Where(item => !string.IsNullOrEmpty(item.GetMetadata(MetadataName.PackagePath)))
-				.GroupBy(item => item.GetMetadata(MetadataName.PackagePath))
-				.Select(x => x.First());
+				.GroupBy(item => item.ItemSpec)
+				.Select(x => x.First()).ToArray();
+
+			//Find files with conflicting package path
+			var duplicates = contents.GroupBy(item => item.GetMetadata(MetadataName.PackagePath))
+				.Where(x => x.Count() > 1)
+				.Select(x => x.Key);
+
+			foreach (var duplicate in duplicates)
+			{
+				Log.LogErrorCode(nameof(ErrorCode.NG0012), ErrorCode.NG0012(duplicate));
+			}
 
 			// All files need to be added so they are included in the nupkg
 			manifest.Files.AddRange(contents

src/Build/NuGet.Build.Packaging.Tests/given_duplicate_package_files.cs

@@ -21,7 +21,10 @@ public void build_fails()
 
 			Assert.Equal(TargetResultCode.Failure, result.ResultCode);
 
-			Assert.True(result.Logger.Errors.Any(e => e.Message.Contains("content.txt")));
+			// content.txt is marked as PackageFile and the path to the file is exacly the same in both project a and b.
+			Assert.False(result.Logger.Errors.Any(e => e.Message.Contains("content.txt")));
+
+			// nuget.js is marked as content and the path to the file is "bin\{projectname}\content\web\js\nuget.js" so to the package they are two conflicting files.
 			Assert.True(result.Logger.Errors.Any(e => e.Message.Contains("nuget.js")));
 		}
 	}