I noticed a post that had the
marked as duplicate by:...
Possible Duplicate: .....
in the question wasn't there, at first I thought it was a bug, but another user informed me that it was removed in an edit.
That surprised me that a user just can edit out or
Rollback to Revision X, to an earlier time.
My suggestion: make the:
Possible Duplicate:... stamp in the answer uneditable. Only by mods or one with a high enough reputation!
The user rolled it back again:
I informed the user to stop rolling back, and re-added the duplicate stamp.