Add a test for #16403

Author: nagisa

src/test/compile-fail/dupe-symbols-8.rs

@@ -0,0 +1,23 @@
+// 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.
+//
+// error-pattern:already defined
+
+
+#![allow(warnings)]
+
+fn main() {
+    {
+        extern fn fail() {}
+    }
+    {
+        extern fn fail() {}
+    }
+}