A common issue that complicates the practice of keeping your iTunes Library cleanly organized is that it's very easy to accumulate duplicate songs in the Library. From iTunes' perspective, a duplicate song occurs any time two or more songs share the same song name and artist. This usually results from the same song by the same artist appearing on more than one album that you've imported into iTunes (a common pitfall of having compilation or "Greatest Hits" albums in your Library).
Normally this duplication doesn't cause serious problems. But having multiple copies of a single song in your Library makes it more likely that the song will come up in randomized (shuffled) playback, whether in iTunes or on the iPod, and it means that your Play Count field is inaccurate because it is split across two or more entries in the database. Also, naturally, duplicates of song files take up precious disk space. It's generally best to avoid having duplicate songs in your Library. You don't have to delete them, though; just disabling the check box on a duplicate song means you can exclude it from Smart Playlists or iPod synchronization, and you will generally listen to only the copy whose check box is enabled.
Choose a Music Source to View
Select the Library view or any playlist from the Source pane. Only the songs within the selected music source are analyzed for duplicates.
Show Duplicate Songs
Choose Edit, Show Duplicate Songs. The song listing changes to omit all songs that don't exist in matched pairs; the only listed songs are those with duplicates within the selected music source, where both the song name and the artist name are identical. For easiest viewing, sort the songs on the Song Name column.
Delete or Disable Duplicate Songs
You can eliminate duplicate songs by deleting one of the copies from your iTunes Library. However, this approach means that you end up with incomplete albums. A better way to filter your music, perhaps, is to simply disable a duplicate song by disabling the check box next to one of the songs. (You might find it more sensible to disable the song that's part of a compilation rather than the one that's on an original album.)
Be careful to eliminate the duplicates only if you're sure they're really duplicates! In many cases, two different songs might have the same title and artist but be completely different recordings of the same song (such as a live and a studio version). You probably don't want to delete one of them in such a case. Be sure to preview each track by playing it briefly before you do anything rash.
Now that your duplicate songs are disabled, you can edit a Smart Playlist that you listen to a lot to ensure that songs where the check box is disabled are not included in the playlist. Do this by selecting the Match only checked songs check box in the Smart Playlist's definition dialog box. (Select the Smart Playlist and choose File, Edit Smart Playlist.) Similarly, in the iPod tab of the iTunes Preferences window (choose Edit, Preferences in Windows or iTunes, Preferences on the Mac while the iPod is connected), enable the Only update checked songs check box. This option prevents disabled songs from being transferred to the iPod and taking up space unnecessarily. It also prevents your Play Count numbers from being spread across two or more identical song files.
Show All Songs
When you're done examining the duplicate songs in your library or other music source, click the Show All Songs button at the bottom of the song listing area or choose Edit, Show All Songs. This command returns you to the normal viewing and navigation mode.