I am receiving the same error message. (I don't think the other two replies understand the issue.) When I got the beta key from Amazon last month, I entered it and it was accepted just fine. (I've verified my email address and have only _one_ account w/ Bungie.) (I was in the Alpha and used the same email etc.) But now the error message tells me (and you) that supposedly the key was, in fact, _used successfully_…but for a different account! I sent a customer service request on the 10th but no reply yet. Sent another request today using a different selected question. Rather frustrating, to say the least. I really would like to play in the Beta and just hope Bungie can figure out what the problem is in time for that to happen.

Unfortunately you'll have to talk to your retailer about this. You are not the first person this has happened to. If you have a smart phone enter the code in front of them. Bungie is not giving out Beta codes, the retailers are the ones doing that.