Duplicated enum entries in the smithy model

[ghostbuster91]

It seems that it is possible to have duplicated enum key-pair entries in the smithy model.

Following snippet passes the validation:

intEnum FaceCard {
    JACK = 1
    JACK = 1
}

The documentation does not say anything about this, there is only a constraint for values to be unique:

Enums do not support aliasing; all values MUST be unique.

I think that is fine, as during the model assembly these two will be flatten into a single entry, but wanted to double check if this is indeed an intended behavior.

Maybe a ValidationEvent(severity: NOTE) should be emitted?

[mtdowling]

This is a bug. We should fail during the parsing phase.