When elaborating predicates, purge duplicates from the initial vector.

Fixes #21965.

src/test/run-pass/associated-types-duplicate-binding-in-env.rs

@@ -0,0 +1,27 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that we do not report ambiguities when the same predicate
+// appears in the environment twice. Issue #21965.
+
+trait Foo {
+    type B;
+
+    fn get() -> Self::B;
+}
+
+fn foo<T>() -> ()
+    where T : Foo<B=()>, T : Foo<B=()>
+{
+    <T as Foo>::get()
+}
+
+fn main() {
+}