Eliminate assoc type projection predicate candidate duplicates

When projecting associate types for a trait's default methods, the
trait itself was added to the predicate candidate list twice: one from
parameter environment, the other from trait definition. Then the
duplicates were deemed as code ambiguity and the compiler rejected the
code. Simply checking and dropping the duplicates solves the issue.

Closes #22036

src/test/run-pass/issue-22036.rs

@@ -0,0 +1,33 @@
+// 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.
+
+trait DigitCollection: Sized {
+    type Iter: Iterator<Item = u8>;
+    fn digit_iter(self) -> Self::Iter;
+
+    fn digit_sum(self) -> u32 {
+        self.digit_iter()
+            .map(|digit: u8| digit as u32)
+            .fold(0, |sum, digit| sum + digit)
+    }
+}
+
+impl<I> DigitCollection for I where I: Iterator<Item=u8> {
+    type Iter = I;
+
+    fn digit_iter(self) -> I {
+        self
+    }
+}
+
+fn main() {
+    let xs = vec![1u8, 2, 3, 4, 5];
+    assert_eq!(xs.into_iter().digit_sum(), 15);
+}