Guilty until proven innocent

Hello there.
In the last few months I recognized an annoying trend regarding my giveaways. Yes, I do blacklist winners due to various reasons. You can think that I'm stupid by doing so, and that's okay. I have my own personal "system", for a lack of a better word, and will hand out BLs relating to my own personal taste.
I can totally understand any winner who is not happy to find out he/she is blacklisted after winning a game, BUT I expect the winner to still be fair and play by the rules.

In the last few months I recognized that winners who I blacklist after their win(s) mark the giveaways as not received in revenge for being blacklisted. The keys are fine. I not only can vouch for that, I even gave my GF some of the "duplicate" keys and they activated fine on her account. So the winners definitely marked the giveaway(s) as "Not received" just out of revenge for being blacklisted.

With the way SG works, it is always up to me, the giveaway creator, to proof that I indeed did sent the key to the winner AND that it was not a duplicate key. I have to try everything in my power to contact the winner to get any kind of proof to send to support. Which is near impossible if the winner is mad at me for having him blacklisted.

I think the system has to be modified. But I don't know how. Maybe the winner who claims the key is a duplicate key also has to proof that the key is indeed a duplicate one? Although I am not too sure how he/she can proof it with a failsafe build into it.
As it is now, any winner always can claim that a won key is a duplicate key and all the trouble then starts for the giveaway creator while the winner - although his claim that the key is a duplicate key is wrong - can sit back and enjoy the experience.