Store Error (UWP): An item with the same key has already been added

Question

I am working on Game Bar widget that includes FT process.
In the beginning I submitted to the store a package that was created by Publish process of UWP C# project.
This app worked locally by the Test package installation BUT didn't work from the store.

I found that probably the package should be based on Windows Application Packaging Project.
So accordingly to Microsoft documentation I added WAP project with Dependency to my UWP project, modified manifest, and created new package.
BUT when I am trying to submit the package(msixupload) to the Store, I get following error:

"Package acceptance validation error: We encountered a fatal error while parsing the package WapProj_1.1.1.0_x64.msix: An item with the same key has already been added.. Try again or upload a new package."

What is the "key" here? What the message is talking about? How can I fix it?

UPDATE:
I noticed that if I remove nuget Microsoft.Windows.SDK.BuildTools from WAP project, store accepts the package.
I do not have a reference to this nuget in my projects.