Oddly in that scenario on Windows there are two clipboard documents alive when
destroying one, so the destroyed one erroneously is said to be a clipboard
source and then detachs the comments' caption information.

There's no legit case where a clipboard document could actually be the real
original source of another clipboard document, so bail out if the caller
already is one.

Change-Id: I0f8d240d8ec13bf91b77176dc6439a541cf14eaa