I've made a huge tiny mistake

So I was giving away all the games I got from the humble bundle. When sending out the keys I accidentally swapped two keys with echother. one of the winner got two keys which one of the keys are the right one. But the other key I sent first belongs to another winner. All I can hope for is that he doesn't redeem that aswell as I told him on the last email. I'm trying to contact the winner that actually won the key that I sent by mistake to the other guy and tell him to hurry up and redeem it. But I can't get any contact at all.

Let's say the person that got both keys by mistake redeems both of them. what can I do then?