A duplicate site ID {0} was found. This might be caused by restoring a content database from one server farm into a different ...

A duplicate site ID {0} 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.