If a shulker bullet manages to hit the shulker it came from, it will create a new shulker in the same way it does with two of them.
Steps to reproduce:
do the setup in the image
keep replacing the shulker whenever it dies
Expected behavior: nothing happens
Actual behavior: new shulkers are eventually created
I'm almost positive that's intended
https://imgur.com/a/tLYtoNe
(Screenshot from https://www.minecraft.net/en-us/article/minecraft-snapshot-20w45a