This error occurs when there are multiple files with the same name in different projects that are being published to the same output directory. For example, in your case, both MyProject.Common and MyProject.WebApp projects may have an appsettings.json file.

To resolve this issue, you can do one of the following:

1. Rename one of the files: If the two appsettings.json files serve different purposes, you can rename one of them to avoid the conflict.
2. Exclude one of the files from the publish output: If one of the appsettings.json files is not needed in the publish output, you can exclude it. To do this, edit the .csproj file of the project and add the following:

<ItemGroup>
  <Content Update="appsettings.json" CopyToPublishDirectory="Never" />
</ItemGroup>

This will prevent the appsettings.json file from being copied to the publish directory.

3. Merge the files: If the two appsettings.json files have different settings that are both needed, you can merge them into one file. Be careful to merge the settings correctly to avoid losing any important configuration.

Remember to backup your files before making these changes to avoid losing any data.