My daughter asked me to clean up her iTunes library. There were lots of duplicate entries in it. So, I googled and found the rinse application by real networks. It claims to eliminate duplicates. Unfortunately, it needs a little work.
My daughter had duplicates of the same file in iTunes. Two entries, one song on the disk. It looks like Rinse doesn't check this. It sees two identical songs and then moves the single song on disk to a duplicates folder.
We had a problem because we share the iTunes folder on our iMac. So, my iTunes now had just 900 songs rather than 5000. Luckily, I went back to my daughters account and reimported the songs from her duplicates folder and everything went back the way it was but it's kind of a silly mistake for the rinse application.
Given all that happened, it's hard to recommend with such obvious flaws.