iTunes Singles Not Credited on Album

This has happened on multiple occassions, and I'm just wanting to make sure this is normal behavior. There has been a few times where I have purchased a single, and a few weeks/months later the album comes out (sometimes as a pre-order). I end up buying the full album, which also has that same song on it, but I'm also charged for that same song and it is basically in my library twice. It's labeled once under the single and then another time under the album. Is this an error on my part or is that normal behavior?