There is a bug pulling in dependencies when targeting multiple frameworks using the "nuget pack" command on a .csproj file. It attempts to add the packages multiple times and fails with an "Item already exists" exception.
Discussion began here:
It turns out this has nothing to do with dependencies, but it had to do with the XML documentation files. The "nuget pack" command was attempting to add both
to the package and that's what was eventually causing the "Item already exists" exception.