After moving the content database to another SQL Server, I started getting the following message in the event log every hour : "A duplicate site ID ... 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."

Nice and simple and even a recommended solution in the error message - except that it wouldn't work. After a lot of additional searching I turned up this command, which did work:

stsadm -o sync -DeleteOldDatabases 0

I am reposting it here because of how hard it was to find in the first place.  For what its worth, the full syntax of this is:

stsadm -o sync
           {-ExcludeWebApps <web applications> |
            -SyncTiming <schedule(M/H/D:value)> |
            -SweepTiming <schedule(M/H/D:value)> |
            -ListOldDatabases <days> |
            -DeleteOldDatabases <days>}

The original answer was posted by Vikram Garlapati.