After rebuilding our farm environment onto a new server with SP1 we started receiving these errors on the hour every hour. It relates to a database that was renamed and thus disassocoated with its web application, then reassociated with it using STSADM -addcontentdb.
Failure trying to synch web application d9ba0571-84d0-40c1-add9-3be5a5a5a443, ContentDB 4606754f-fb82-4829-bda1-878869c2a450 Exception message was A duplicate site ID 18318f24-21f6-4b45-95c3-5479fe6f99dc(http://ssp4dev/ssp/admin) was found. This might be caused by restoring a content database from one server farm into a different server farm without first removing the original database and then running stsadm -o preparetomove. If this is the cause, the stsadm -o preparetomove command can be used with the -OldContentDB command line option to resolve this issue.
stsadm -o sync -DeleteOldDatabases 0
Refer to MS for the full STSADM sync command info.
stsadm -o sync