I just wants to make sure that the key goes to the person who doesn't have this game and he deserves the prize. Ultimately(!), the winner will have to accept my friend request. After verification, of course, if he wants, he will remove me from his friends list.

No, what I do are not custom rules. Simply put, the winner of the giveaway should activate the won game on their Steam account (linked to SteamGifts) and if they already have it, they should not register. And as the author of the giveaway, I simply check whether the potential winner deserves the prize.
So I simply act according to the rules of the Border Guard. If you think otherwise, report me. No problem. I guarantee you that SG support will agree with me.

PS. If you want to participate in SG giveaways, you must synchronize SG with Steam at least once a week. If there is information in the SG database,
if you have a AAA game, you cannot enter giveaways with this game. But if you have this game on Steam but you don't sync SG with Steam, you will still be able to enter giveaways with the AAA game. And users have a window of maximum one week in which they can actually (unconsciously) break the regulations of this website.
And that's why I want to personally verify whether the potential winner deserves to win.