Auch, that hurts even to hear this. I noticed the problem few times... I even caused it once after activating over 100 games in one day... I won about same time when I activated game or maybe I missed one game when searching entered games - hard to tell. It was only one or two minutes difference :)
But yeah, I understand SG lack proper tool for auto mass reroll, quick check while selecting winner... or button "I already have it"... some entry cooldown after reroll... or maybe just more aware users. Unfortunately most of people asking for reroll don't read comments/descriptions.
And if you are really unlucky, you may need few waves of rerolls, Once I had giveaway with several entries but only one entrant didn't have the game xD But that was before update removing entries after winning one lottery. Perhaps similar solution could be implemented after sync...

Well, I hope you get somewhat less rerolls ^^

SG automatically exists all copies of entered giveaways the moment you win (even if a win isn't delivered).
It happens that people forgot what they immediately won and enters it again before the next scheduled sync (unless they manually sync).