Accept a duplicate call, but nothing else.

Change-Id: Iec24bf5ddc3b0f0c559ad2158339aca698601743
Reviewed-on: https://go-review.googlesource.com/23692
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>