Hi Elkin,
This appears to be duplicate post - simliar query of yours is also handled at following thread; http://scn.sap.com/thread/3480176.
As u say, workflow can help you find the duplicate and overwrite the other fields that are duplicates or needs a change. This can be achieved assigning the appropriate workflow and assigning the same in Import manager configurations option.
Pls chk and let us know if any queries.
Regards,
Ali