In iTunes, it’s very easy to end up with duplicate songs. There are many ways this happens, but recently many people are ending up with duplicate songs as they upgrade their old iTunes Store purchases for new iTunes Plus tracks (see below about this). There are several ways to permanently remove duplicate songs from iTunes. This will save hard drive space, and can make it easier to browse your iTunes library.
It’s worth noting that some tracks appearing to be duplicates may actually be different songs; for example, a live version vs a studio version. Be careful not to delete these. Otherwise, it’s very easy to remove duplicate items in iTunes with built-in management tools.
Apple has a knowledge base article about this which you can read by clicking here.
Some people use third party apps or applescripts to weed out duplicate tracks. Take a look at Tidy Up!: http://www.hyperbolicsoftware.com/TidyUp.html and Dupin: http://dougscripts.com/itunes/itinfo/dupin.php
Also, here’s a virtually unknown keyboard short cut: in iTunes, hold down the Option key and click “File”. This will change the menu entry to “Show Exact Duplicates”. Show duplicates only displays tracks with identical titles, but selecting “exact” will only display tracks where ALL fields are identical.