iDupe lets you manage your duplicate files on iTunes. This application can scan your iTunes library and determine what files are duplicates and what files are dead tracks. The main window consists of five tabs that let you customize the scanning process, see the reports after a scan, and some help for using the application. From the "scan settings" tab, you can change all the settings that have to do with the scanning of duplicates. You can set how the app scans for dupes and what it does when it finds one. There are many different settings, but, for example, you can set iDupe to keep the newest track found when a duplicate is found, or to keep a song that has been rated. It also can consider bit rates when dupes are found, keeping the highest one.
To start a scan, you first have to launch iTunes and select the songs that you want to scan. Then go back to iDupe, and in the "Actions" tab, start a scan. After it is done, you will see a text report with all the cases of duplicates the app has detected. You can now start taking action. You can remove the duplicates or simply move them to another playlist.
I guess an app like this could come very handy with large iTunes libraries. I only have like 200 songs, and the app didn't find anything worth mentioning.