Message 381018 - Python tracker

@Guido and @Ivan,

Should Literals de-duplicate values in its __args__ similar to Union? PEP 586 states that:
> Literal[v1, v2, v3] is equivalent to Union[Literal[v1], Literal[v2], Literal[v3]]

Naturally, a Union de-duplicates values, so I was wondering if Literal should follow.