How to find and remove duplicate items in your iTunes library - http://support.apple.com/kb/HT2905
post by turingtest2 - https://discussions.apple.com/thread/3555601 - different types of duplicates and techniques
http://www.araxis.com/find-duplicate-files (commercial, free trial) - finds duplicate files on computer, not specifically iTunes
I would do it manually. As you can see from turingtest2's post, some duplicates are not really duplicates.
I would also review how you add media to iTunes. It should not be producing all that many duplicates.