OK, well, apparently these things happen. I personally don't know exactly why, but apparently even if you get a key from Indiegala or Groupees or some other site, it can be a duplicate. It's happened before. Anyway, it's a dupe, so I wasn't able to redeem it and I can't mark it received. But if you want you can create a ticket and ask support to delete this giveaway and that way my "not received" won't affect your ratio and anyone who looks at your profile won't think that you just didn't deliver because you're shady or something.

Oh yeah, and I guess if you already have the game in your library, you can't check a key for whether it's a dupe or not. The "you already own this game" message overrides the "this key is a duplicate" message.