Breaking a door with a pickaxe returns two doors, each linked to a different dimension.
attached are all the loaded mods.
modlist.txt

Is there a way to fix this? As far as I know the door only duplicates when you break it in survival mode. (In creative mode it drops only one door)

Broken doors still duplicate in 1.16.4, however both doors are linked to the same dimension.
when you leave the dimension while it has two doors, you go to the last one placed, enabling you to quickly teleport large distances...

you can also do this by placing the duplicated door inside of the dimension, which is fun.

I'm kinda suprised none of this crashes

also you cant leave if you place the duplicated door inside lmao

from in-game testing this appears to be because the door is technically present in both dimensions, and both are broken when you break it, resulting in two drops.