I'm having a similar problem with Hearthfire. I bought it, download froze at 78%. So I stopped the download, and it says I have to purchase it again. So now I have a partially downloaded Hearthfire on my hard drive, and it says I need to buy it again. Also, It removed 800 MS points from my account, which makes no sense as I only clicked the confirm purchase button once. I get an error when going into my download history and I can't see my gold subscriptions on the Account tab on the Dashboard. I'm just going to wait it out, hopefully it all gets resolved. I'm still kinda pissed.
Try now. I just restarted my Xbox to see the "Download Again" option instead of the "Purchase" one.
almost got a bit worried that I'd have to buy Dawnguard again...