I'm on the fence on this one, but maybe the flag dialog should show the message as it was at the time of the first flag, instead of the current version?

Or, alternatively, at least provide some immediate indication that the message had been edited.

In some cases I guess people could legitimately be correcting their mistake, but they probably shouldn't be able to dissuade subsequent flaggers by quickly disguising what they wrote in the event their intent was malicious.