Checking for non-activation and multiple wins?

I want to make sure my winner isn't cheating by joining giveaways for a different account / for his friends. Some people don't care, but I do, so I check.

From FAQs:

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.

When should I mark a gift I won as "Received" or "Not Received"?
You should only mark a gift as "Received" when you have both (i) received the exact game described in the giveaway, and (ii) activated that game on the Steam account you used during registration. In all other instances you should mark the gift as "Not Received" one week after the giveaway has ended.