Filter out duplicated trait predicates when generating auto traits

Fixes #51236

src/test/rustdoc/issue-51236.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+use std::marker::PhantomData;
+
+pub mod traits {
+    pub trait Owned<'a> {
+        type Reader;
+    }
+}
+
+// @has issue_51236/struct.Owned.html
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//*/code' "impl<T> Send for \
+// Owned<T> where <T as Owned<'static>>::Reader: Send"
+pub struct Owned<T> where T: for<'a> ::traits::Owned<'a> {
+    marker: PhantomData<<T as ::traits::Owned<'static>>::Reader>,
+}