I noticed a post that had the
marked as duplicate by:...
But the
Possible Duplicate:
.....
in the answer 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!
EDIT:
The user rolled it back again:
I informed the user to stop roll backing, and re-aded the duplicate stamp.