This is a DLC. If I understand matters correctly, because of how steam reports owned apps (and maybe partly how SG gathers that information), SG really cannot tell who owns what DLC. Only steam and steam friends can tell.

Thus, the only sure way (when giving away a DLC) to make sure that your winner does not have it in their inventory is to friend them on steam long enough to check [eg trying to purchase the DLC for them (but not purchasing it, of course) or going to the DLC's store page and seeing if they have it in their account].