# Generate error for duplicate files in publish output

The .NET SDK generates a new error (NETSDK1152) in cases where files from different source paths would be copied to the same file path in the publish output. This can happen when a project and its project references include a file with the same name that's included in the publish output.

.NET SDK 6.0.100

## Old behavior

Both files were copied to the same destination. The second file to be copied overwrote the first file, and which file "won" was mostly arbitrary.

In some cases, the build failed. For example, when trying to create a single-file app, the bundler failed with an ArgumentException, as shown in the following build output:

C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Publish.targets(962,5): error MSB4018: The "GenerateBundle" task failed unexpectedly. [C:\repro\repro.csproj]
C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Publish.targets(962,5): error MSB4018: System.ArgumentException: Invalid input specification: Found multiple entries with the same BundleRelativePath [C:\repro\repro.csproj]
C:\Program Files\dotnet\sdk\5.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Publish.targets(962,5): error MSB4018: at Microsoft.NET.HostModel.Bundle.Bundler.GenerateBundle(IReadOnlyList1 fileSpecs) [C:\repro\repro.csproj]


## New behavior

Starting in .NET 6, MSBuild removes duplicate files that are copied to the publish folder if both the source and destination are the same. If there are any remaining duplicates, a NETSDK1152 error is generated and lists the files that are duplicated.

## Reason for change

Duplicate files in the publish output sometimes caused build breaks or unpredictable behavior.

• Ideally, you should update your project to avoid situations where multiple files with the same name are copied to the publish output. The error message includes the name of the duplicate file. Some causes for duplicate files include:

• An ASP.NET Core project that references an ASP.NET Core web service, and each has its own appsettings.json file.
• A project item where CopyToOutputDirectory is unnecessarily set to Always.

Binary log files can be useful for finding the cause of the duplicated files.

• Alternatively, you can set the ErrorOnDuplicatePublishOutputFiles property to false`.

N/A