Since you viewed the key, a re-roll is likely to be denied (I had a conversation with support about this the last time it happened), and if it is accepted then it is likely to place the burden of responsibility back on the giveaway creator (me!). I think support may have changed how they deal with this after cg implemented the feature where keys aren't automatically made visible to the winner.

Unfortunately, after a spot of research I don't know the best solution for this other than for you to accept the key (I don't think this will cause any problems for you, since you didn't win it twice on this site).

If you know of any better ways to deal with this, I'm open to suggestions. It's very easy to make a mistake these days when checking that you don't already have DLC (or indeed a game with Limited Profile Features on Steam), and it's not my intention to cause any difficulty (I just want to avoid any further difficulty for myself!). I know that I'm still trying to remind myself to double check before i reveal key a key when I win a giveaway...