Can define a associated type more than once

@eddyb

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.

triage: I-nominated

classifying as just a bug. P-backcompat-lang, not 1.0 blocker.

Demoting this to P-medium. Somebody should get on it though!

Better detection of duplicate associated items.

Rollup merge of rust-lang#25993 - nham:fix_23969, r=nikomatsakis

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