Happend multiple times but im not sure how to replicate it tho.
However it definetly has to do with rejoining the world.
I had a shield in my offhand and when i rejoined, another shield appeared in my inventory
also pretty sure some other items can duplicate aswell because i remember a sword doing so.

I believe this happened because of mobs being able to pick up items on their own during possession, which should be fixed in 2.0.0-beta.11. Please open a new issue if you can still reproduce this duplication.