Explicit duplicated types in inheritance should error

[dburgener]

This currently compiles fine:

virtual resource foo {}

virtual resource bar {}

resource baz inherits foo, foo, bar {}

domain requires_allow {
        allow(this, baz, file, read);
}

It should error and tell the user that "foo" was listed twice when inheriting baz.

On the other hand, something like this:

virtual resource foo {}

virtual resource bar inherits foo {}

virtual resource baz inherits foo {}

resource qux inherits bar, baz {}

domain requires_allow {
        allow(this, qux, file, read);
}

Compiles fine and should continue to do so, because the duplication isn't explicit. Since qux inherits bar and baz which each inherit foo, that's fine, and the compiler should internally dedup and only inherit foo once.

The second case is working correctly today, but we want to ensure that it continues working when we cause the first case to error.