Do I have to buy this song again?

A while back I downloaded a song from one album and everything is great. I found another album on iTunes that has many songs I would like to purchase. The catch is that I have to purchase the entire album. I can't just pick and choose the songs I want off of the album. One of the songs in the album is the one I purchased a while back. My question is, what will happen if I buy this album? Will I have two copies of the one song? If yes, then is there a way to not have this happen, other than not purchase the album.