You have mail archive enabled and something unexpected happened wrt to database queries. There's two places that message is generated. One is in searching for a particular message ID and getting more than 1 hit back (searching on a unique ID should give 1 record...or 0 if a user does not have access to view it).
The second place that happens is when you try and restore a message...it's basically the same thing as above...you've clicked on "Resend Message" and instead of finding a single record (or none because of ACL), more than 1 record was returned.
As for a solution...I couldn't begin to tell you how to go about solving...it will require some debugging on your part that will definitely involve looking at the MySQL database for the archive_current and archive_search databases. See
the user manual for information on how to access those databases.
Ben
