Repeating idents in functional dependencies crashes compiler

[PROMETHIA-27]

Minimal repro:
trait Crashy a -> t t

This is sufficient to cause the panic:
thread 'main' panicked at 'assertion failed: prev.is_none()', src\nameresolution\scope.rs:172:9

[jfecher]

Fixed in 056b108.

This is somewhat related to the previous change with resolving trait arguments. The fix is to change this assert to instead push a user visible error that the type variable was declared twice. These type variables don't actually have Locations associated with them though, so the reported location is only approximate and can be improved.