Symptom: You've got a mess of duplicates in your iTunes library.
Cure 1: This can occur when you've used the Add to Library command one time too many. To start fresh, select everything in your iTunes library (the one in iTunes) and press your keyboard's Delete key.
When you're asked whether you want to delete the files from your hard drive, respond NO. We're simply clearing out the list of songs from iTunes, not deleting the songs from your hard drive.
Open iTunes' preferences, click the Advanced tab, and make sure that the Copy Files to iTunes Music Folder When Adding to Library option is unchecked. Now select Add to Library from iTunes' File menu (or Add Folder to Library if you're using Windows), navigate to your music folder (called Music on the Mac and My Music on the PC) and click Choose. All the songs in the folder will be added to iTunes. If you want to grab all the music on your computer, select a directory higher up in the file hierarchy -- your Mac's startup volume or your PC's C drive, for example. iTunes will add any compatible music and audio files to its library.
Cure 2: If you're using a Macintosh, use a helpful ApplesScript to remove the duplicates.
One such AppleScript is Remove Duplicates, which removes any duplicates spawned by the same audio file (in other words, you only have one copy of the file but multiple entries for it in iTunes).
Another is Corral All Dupes. This one searches for songs that appear to be the same and places them in a playlist, allowing you to decide which to keep and which to delete. These duplicates are multiple copies of a song -- perhaps one in MP3 format and another in AAC format.
Both scripts are from Doug's AppleScripts for iTunes site.
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