[Feature Request] Warn about duplicate types in `Variant`

[soraros]

Review Mojo's priorities

• [X] I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

As title.

What is your motivation for this change?

One of the branch is dead code anyways.

Any other details?

The problem is if we do it on the library level (meta-programming + something like _UnionSize) or on the IR level?