diff --git a/src/librustc/middle/traits/util.rs b/src/librustc/middle/traits/util.rs
index 45ce692bb076..6cbac7d01e7b 100644
--- a/src/librustc/middle/traits/util.rs
+++ b/src/librustc/middle/traits/util.rs
@@ -11,12 +11,12 @@
 use middle::subst::{Substs, VecPerParamSpace};
 use middle::infer::InferCtxt;
 use middle::ty::{self, Ty, AsPredicate, ToPolyTraitRef};
-use std::collections::HashSet;
 use std::fmt;
 use std::rc::Rc;
 use syntax::ast;
 use syntax::codemap::Span;
 use util::common::ErrorReported;
+use util::nodemap::FnvHashSet;
 use util::ppaux::Repr;
 
 use super::{Obligation, ObligationCause, PredicateObligation,
@@ -36,7 +36,7 @@ use super::{Obligation, ObligationCause, PredicateObligation,
 pub struct Elaborator<'cx, 'tcx:'cx> {
     tcx: &'cx ty::ctxt<'tcx>,
     stack: Vec<StackEntry<'tcx>>,
-    visited: HashSet<ty::Predicate<'tcx>>,
+    visited: FnvHashSet<ty::Predicate<'tcx>>,
 }
 
 struct StackEntry<'tcx> {
@@ -65,14 +65,11 @@ pub fn elaborate_trait_refs<'cx, 'tcx>(
 
 pub fn elaborate_predicates<'cx, 'tcx>(
     tcx: &'cx ty::ctxt<'tcx>,
-    predicates: Vec<ty::Predicate<'tcx>>)
+    mut predicates: Vec<ty::Predicate<'tcx>>)
     -> Elaborator<'cx, 'tcx>
 {
-    let visited: HashSet<ty::Predicate<'tcx>> =
-        predicates.iter()
-                  .map(|b| (*b).clone())
-                  .collect();
-
+    let mut visited = FnvHashSet();
+    predicates.retain(|pred| visited.insert(pred.clone()));
     let entry = StackEntry { position: 0, predicates: predicates };
     Elaborator { tcx: tcx, stack: vec![entry], visited: visited }
 }
diff --git a/src/test/run-pass/associated-types-duplicate-binding-in-env.rs b/src/test/run-pass/associated-types-duplicate-binding-in-env.rs
new file mode 100644
index 000000000000..62ac21879520
--- /dev/null
+++ b/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() {
+}