NOVEL WEB OFA Explanation (NAC)
So! I'm just going to be explaining how I deal with One For All.
No, it won't kill the person if they already have a quirk. Because then Izuku or Melissa would have to take it. Instead, I'm taking inspiration from Clouds over on Ao3.
The person who inherits OFA will obtain some enhanced strength, which will increase the more in-tune they become with the quirk. Or how much they train their body. Wouldn't be OFA if it didn't make the person outright stronger.
But the important part is that the stockpiled quirks will manifest much more easily depending on who inherits it.
Depending on their quirk, the stockpiled quirks adapt to suit it. If they can't (read, I couldn't think of something) they'll manifest much later as themselves, if they manifest at all. They also won't be nearly as powerful as if they'd integrated.