If you've cleaned up your iTunes library using Duplicate Cleaner, you may notice that the tracks are still shown upon starting iTunes. This is because even though they have been deleted from the hard drive, they are still in the iTunes database. If you try to click to play one of the removed songs, it will fail because the file has gone.
You could remove these 'dead' songs from your library by hand, but it could take a long time. The fastest way to do it is to use a script, which Apple provides.
Download the RemoveDeadTracks script here.
Unzip the script somewhere (ideally in your documents folder)
*Close iTunes
Double-click the RemoveDeadTracks.js file to run the script
iTunes will now open and start removing the dead tracks. You may not notice anything happening, and it could take a little while, depending on the size of your library. If you look in the Song view, you should see the 'items' count reduce at the bottom of the window.