Duplicate predicates cause ambiguities when normalized associated types

[nikomatsakis]

The following example fails to compile:

trait Foo {
 type B;

 fn get() -> Self::B;
}

fn foo<T>() -> ()
 where T : Foo<B=()>, T : Foo<B=()>
{
 <T as Foo>::get()
}

fn main() { }

It yields:

<anon>:10:5: 10:21 error: type annotations required: cannot resolve `<T as Foo>::B == _` [E0284]
<anon>:10 <T as Foo>::get()
 ^~~~~~~~~~~~~~~~
error: aborting due to previous error
playpen: application terminated with error code 101

However, if you comment out the duplicate where clause, it will compile just fine.