I rented a movie on iTunes and an error message has shown and won't let me transfer it to my iPad

I rented the film 'The Dark Knight' the other day in HD and now it has downloaded onto my Netbook and whenever I attempt to move it onto my iPad it says 'The move 'Dark Knight' could not be transferred because it is already been transferred to another computer or device, but I only downloaded it onto my Netbook. This took me 4 hours to download and now i can't watch it, please help me, and if you can, can you tell me why this is?