I just won 2 copies of Mirror's Edge what do I do?

You should decline one of them, and advise the gifter to request a re-roll :)

Re-gifting and winning the same game twice are two surefire ways to get up close and personal with the suspensionhammer...

Do what the posters said above me.

I know some users that have been banned for keeping more than one copy of the same game.
How do I know? Because I reported them for that reason, and then checked days later theirs profiles... ;)

Damn bro, you're lucky!

I wouldn't doubt that a quarter or more of the massive amount of origin bundle game giveaways on this site are actual Origin keys, and just the giver and winner aren't mentioning it. SG could just legitimatize giveaways of their keys if they wanted, Gala did.

How do I tell which one it is?
I think Origin keys from that bundle are one segment longer than Steam keys, but I'm not sure. Someone who bought the bundle should be able to say.

keep the steam key (and click recieved for that giveaway), report the guy that sent an origin key, easy as that.

Or you can trade with me for another key from the Humble Bundle so its a win/win for you. If you and I dont get a suspension lol.

Pick one to keep and one to message the giveaway guy back tell him you won two copies and ask him to re-roll that one.

It is good that you are being honest about it. May you keep geting blessed with good karma and many more wins.

If you don't know which key is from which giveaway just respond to mail with supposed origin key explaining that you won second one at same time and ask for reroll. When it gets rerolled you will have only one giveaway to mark as recieved.