I officially goofed I think...

i think you understood already that you are buying another copy of the game... :/ i'm afraid...

Contact the user. If they can convince you/support that the key is unused, you should be able to get a re-roll.

If we're talking about the Dungeon Defenders collection, they may have entered for the DLC. While it's good etiquette when you own the base game to check this is ok with the gifter, many don't. It could even be an honest mistake, where he/she bought the game and forgot to remove their entry.

Moral of this: Always check your winners and, where possible, try to give keys via Steam chat, so that they have the chance to decline, or explain themselves...

Give the key to a friend to use. As soon as possible. Can't reroll it for the reasons already mentioned.

How do you know the game in the account isn't from your key, since you didn't check before you sent it?

Report him only if you're certain he DID NOT break any rules. Give the key to friend to try it - if it works = awesome. If not, it was activated and... well, shit happens.

Are you talking about Dungeon Defenders Collection?

With base game in his library he entered only for DLC.

You didn't set special rules, so basically there is no criminal :)

This games is bundled, no big deal.

Never send key by e-mail, use steam chat.