Add test for fix duplicate display of E0502

Author: Josh Leeb-du Toit

src/test/compile-fail/issue-42106.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 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.
+
+fn do_something<T>(collection: &mut Vec<T>) {
+    let _a = &collection;
+    collection.swap(1, 2); //~ ERROR E0502
+}
+
+fn main() {}