Well, in addition to any duplicates, a record is also related to itself. You want to be looking for Count > 1.
Let me suggest another way to handle this. Make your "Unique" field a Text field, with auto-entered calculation (replacing existing data), using the same concatenation formula. Set the field's validation to 'Unique, Validate always'. This way, duplicates will not be imported to begin with, so you don't have to hunt for them.