@ kisjsijxjjdiejne Izuku's Quirk, One For All, is a transferable Quirk that can be passed on from one user to the next. It was transfered to some people that already had a Quirk, like the 3rd and 4th user. It was a somewhat unique circumstance of someone already having a Quirk and getting a new one on top of their own Quirk, and without All for One's doing. And later on, the core of One For All has grown in strength from being passed from user to user, and the Quirk Factors of all of the previous users have merged into the core. So Izuku Midoriya now has the exclusive ability to access the Quirks of previous One For All users.
Gaining multiple Quirks often puts immense strain on the inheritor, rendering them as mindless living dolls incapable of speaking. It was initially believed a person can tolerate two Quirks at most, should they be physically fit, as was the case with the One For All users, but it turns out they too suffered from the backlash of carrying two Quirks, which resulted in a significantly decreased lifespan.
There are only three known people who have been able to wield multiple Quirks without suffering from this Quirk overload, All For One, Gigantomachia, and Izuku Midoriya, and three who have been able to achieve this feat through heavy modification on the body and mind, Number 6, Nine, and Tomura Shigaraki. It is potentially possible that a Quirk that can heal the body or provide more energy to the user can supersede the negative effects of holding multiple Quirks, as seen through Gigantomachia's Endurance, Tomura's Super Regeneration, and Nine's attempt to claim Cell Activation.