How to clean up duplicates in iTunes library?
Apple doesn't have anything natively to select duplicate files so, unfortunately, you'll have to resort to a paid solution like Dupin or you could try Randy's Träume's script.

I haven't used either so I can't vouch for how well they work, but forums I found relating to this problem point to Dupin frequently so that might be your best bet if you don't mind spending a little coin.