One Character, One Body
A character can no longer be in the world twice at the same time.
The same character could be logged in from two connections at once, each with its own body. That is the classic way to duplicate items: drop them with one body, leave with the other, and whoever saves last erases the loss.
A character is now locked while it is in the world, and unlocked the moment it leaves — including when the connection drops, so nobody is ever shut out of their own character.