Issue a warning for duplicate items (Compile, Reference, ProjectReference)

I think MSBuild tooling should issue a warning if it encounters duplicate items such as Compile, Reference or ProjectReference. We've had bugs in Roslyn not handling duplicate items properly and I'm imagining there could be other negative repercussions from having duplicate items.

We've thought about adding the warning to the project system but it feels like they should be issues uniformly regardless of whether you're using command line build or IDE build.

There are separate bugs to harden Roslyn against duplicate items and do deduplication on all entrypoints.