Can define a associated type more than once

``` rust
trait Foo {
    type Ty;
}

impl Foo for () {
    type Ty = ();
    type Ty = usize;
}

fn main() { 
    let _: <() as Foo>::Ty = ();
}
```

This erroneously parses and compiles fine, the compiler seems to use the first definition of `Ty` in the impl.

This might have been introduced by #23265, ping @eddyb.
