The larger your music collection, the more likely it is to contain duplicate songs. For instance, I own Billy Joel's Greatest Hits, which includes songs from various other albums that I also own. Consequently, my library contains two instances each of "Piano Man," "My Life," and so on.
Fortunately, iTunes has a secret weapon against duplicate songs:
1. In the Library section, click Music.
2. Click File, Show Duplicates.
3. Presto! You'll see all the songs that have the same name.
To remove a duplicate, just right-click it and choose Delete. Note that doing so merely removes the song from your iTunes library--it doesn't delete it from your hard drive.
Also, before you start deleting songs willy-nilly, make sure they're identical. For example, Sting's "Moon Over Bourbon Street" appears twice in my library, but that's because I have both studio and live versions of the song-and I want to keep both.
If you're not an iTunes user, try MediaMonkey. This shareware classic makes simple work of finding and removing duplicate songs from any music library.