As Artie mentioned, the App store catalogs over 100 apps for finding duplicates (use the keyword "duplicate" as a search term). Several are specialized for iTunes and iPhoto libraries while others are more general in application. I have had good results in iTunes using iDeDuper, it has been around for a while and keeps improving. "Any Duplicate Finder" is more generalized and has specific functions for scanning, music, photos, and movies. With over 100 apps to choose from the only issue you are likely to encounter is whether or not an app is compatible with 10.6.
