I wish I remember where to read this tip from, so I can give proper credit. To delete duplicate songs in your iTunes library, first select your library, then click on the Edit / Show Duplicate Songs menu. At this point, iTunes will show you all songs that it thinks duplicate, sorted by name.
Before you delete them off the library, right-click (or Ctrl+click) on the song and choose Show Song File to locate the file in the Finder. Delete both from the Finder and the iTunes library.