Here's a couple of examples:
Dupin (also Dupin lite)
there's other apps if you search for them. I haven't seen one that's free, although I may have missed that.
If you have only a small number of duplicates, then open your iTunes, hold your Option key, and drop the File menu. Choose Display Exact Duplicates. And, then you delete duplicates from that list. Be careful, as the list will show BOTH duplicates. So, you would delete only one dupe (or leave the last one if there's a large number of identical files.)