Forge 36.1.0
MC 1.16.5
Quark r2.4 309

I made the discovery that an infinity water bucket is able to be duplicated by having the water bucket constantly be swapped by an empty bucket and vice versa. This seems to be the most effective when putting the water bucket in the hotbar, and the empty bucket right above in the inventory.

Doing this with a stack of buckets ends up with it becoming one infinity water bucket.

I've included a .gif to showcase the bug.
https://imgur.com/3xxXel0

Reproducible in 1.16.5 r2.4-311