Won a game thats already activated ?

If the key is duplicated, contact the giveaway creator.

You can mark the gift as "Not received" provided that you don't reach an agreement in 7 days

Wait till the seven days have passed and mark it as not received.

1. Leave a message as a comment in the giveaway explaining the problem with the key.
2. If the giveaway creator hasn't resolved the issue within 7 days of the giveaway's end, mark the giveaway "Not Received."

The creator may ask your permission to delete the giveaway. That's up to you, of course.