Little more info needed.
Are you trying to use the report as a log of duplicates? Or are you trying to exclude duplicates and show only non-duplicates?
If you're trying to just display duplicates in the same table, couldn't you use a Duplicates Query (through the New Query Wizard) to isolate these, and then set that query as the report's Record Source? Is there something else you need done? Because that should, in effect, do what I think you're asking for. It would only show records (based on criteria you know would be the same, for example, a part number) that are exactly alike in the same table.