Last modified: 2011-03-13 18:05:24 UTC
While it might be a good idea to mark versions as quality for multiple times, due to the comment box, there is no similar reason for sighted versions. Thus, it would be nice to show an error message if a version that is already sighted is flagged again in an identical manner.
This is already the case if you try to flag an unflagged version as unflagged again.
Not sure what an error would accomplish. Also, some websites may have a use for sighting multiple times, in addition to removing accidentally sighted template/image vandalism or such.