This notably fixes the newest "Donkey Dupe", but also fixes a lot
of dupe bugs in general around nether portals and entity world transfer
We also fix item duplication generically by anytime we clone an item
to drop it on the ground, destroy the source item.
This avoid an itemstack ever existing twice in the world state pre
clean up stage.
So even if something NEW comes up, it would be impossible to drop the
same item twice because the source was destroyed.
This should make us more forward proof on preventing dupes.
These dupes have been in for years at this point, they aren't new...
Everyone knows about them and are mitigating with plugins atm breaking gameplay.
so better to make it clear its fixed in the messaging.
I am submitting this to Mojang.
