what if i have a song thats on a cd i'm downloading?

you mean that you have already donwloaded that song from iTunes and now you want to download the other songs?

in this case you just click on Complete My Album, and you'll download just the songs are missing, and you'll pay for the difference between the songs you've already downloaded.

But if you have a song which was not purshased from iTunes Store, I guess it'll be downloaded again, and of course you're going to pay for that.