This already happens with Songs. My daughter bought a song but it did not download properly, so she pressed it again and got the message "You have already bought this song, do you want to buy it again?".
No I fucking do not. I want to download it as I've already paid for it. I went to the computer and tried it there as well. It won't download the song as part of the purchased ones, and insists I buy it again.
I fucking hate Itunes.