Previously, if you bought single tracks on iTunes and then decided to
buy the album, you effectively paid twice for the songs you already
owned.
Now, you can use Complete My Album up to 180 days after first
purchasing individual songs from any qualifying album to purchase the
rest of that album at a reduced price, i.e. the cost of the album minus the songs you already own. Thanks, Apple! http://www.apple.com/pr/library/2007/03/29itunes.html