It's actually easier to filter out duplicates all together than to allow the record to import and then flag the duplicate...
I haven't tried this during an import, but if you define a self join on the target table matching by first and last name fields, the duplicate table can be defined to auto-enter:
Count ( TargetTable 2::firstName )
This requires enabling auto-enter options during import. There's a check box for that in a small dialog that appers just before the import starts.