The bug

As long as the player is dead, multiple entities can spawn with their UUID.

Reproduction

This will require you to obtain your own UUIDLeast/UUIDMost pair. You can use the following commands in the chat to get these values:

/data get entity @s UUIDLeast
/data get entity @s UUIDMost

First, place the following command, replacing the UUIDLeast/UUIDMost with your own, in a repeating command block and turn it on. Note in the command block output as well as the game output log that the entity couldn't be summoned due to the player already having the specified UUID.

summon creeper ~ ~1 ~ {UUIDLeast:-9192300185346116154L,UUIDMost:-5068141680530666168L}

Now type the following in the chat:

/kill @s

After your player's death animation, a multitude of creepers will start spawning until you click "respawn". These creepers all have the same UUID as each other, being the same as the player. Under normal circumstances, an entity cannot be created if it would share the same UUID as another entity.