Warn about duplicate dependencies in package definitions

[rgrinberg]

Currently, an opam package maybe defined as:

(package
 (name foo)
 (depends ocaml ocaml))

And dune will let the above pass through. We should be a bit more diligent here and make sure duplicates are forbidden.

[ghost]

That seems reasonable

[benodiwal]

Hey @rgrinberg, I have raised a PR for this issue. Thanks