How Do I Find And Delete Duplicate Songs In iTunes?
This option was removed in iTunes 11, but has been added in iTunes 11.0.1. Make sure you have the newest version by going to the iTunes menu and selecting "About iTunes". If you're not seeing 11.0.1 or newer, go to the Apple menu to Software Update for the newest version.
Once you have iTunes updated, go to the View menu and select "Show Duplicate Items" to see a list of each duplicate in your library. You can then go through item by item and delete anything you don't need.