iTunes has long offered a basic feature for identifying duplicate tracks, however when iTunes 11 was initially released, many users were surprised to find the feature had gone missing. Regardless of whether this was intentional or an oversight on Apple’s part, it quickly returned the feature in an update to iTunes 11.0.1.
If you’re missing the Show Duplicates feature, be sure you’ve upgraded to the latest version of iTunes, and then take a look on the View menu. The feature itself works in much the same way as in prior iTunes versions, filtering your list view down to only suspected duplicate tracks and then letting you deal with this information as you see fit.
Keep in mind that for music tracks, however, this is based only on the track name and artist, so if you’re a collector who wants to keep multiple, complete albums you may find that you always have duplicates in your library as far as iTunes is concerned. When viewing duplicates, a status bar appears at the bottom of the iTunes window to remind you that you are in this mode, along with a “Display all” button that can be used to return to the full track view.