Right. You need to contact their support and let them know the key was a dupe, they should give you a new key.
If they for some reason refuse or can't provide a new key for this game, you have a few options:
Get a new key from somewhere else.
File a support ticket and ask to have the giveaway deleted. (they will approve as long as I agree to it, which I would do.)
Just ignore it and take the "not received".