From the FAQ:

"What should I do if I win a game more than once, or if I win a game that I already purchased?

When this occurs, you should contact the giveaway creator and inform them you will not be accepting or redeeming the gift. Explain the situation, and ask the giveaway creator to request a new giveaway winner. During this process you should not mark the gift as "Received", or reveal the key if the option is available. This will help inform us that you are not accepting the gift, and it will prevent you from being suspended for not activating a gift you won. Once the ticket from the giveaway creator is approved for a new winner, the win will be removed from your profile."

I believe what you have to do is go to "Support", create a support ticket, select "Request New Winner" as the category, and then fill out the required information.

Sorry for the inconvenience btw, not sure why it let me enter.