You can very easily duplicate items, all that's needed is two players
Version, Fabric 1.20.1

First player, (will refer to as A) places down a shop, giving it stock and setting any price.
Second player, (will refer to as B) then enters the shop GUI
A then breaks the shop block, which drops out any stock inside of it and the shop item
However player B is still inside the shop GUI, the GUI has NOT closed when the shop block was broken
Player B can then buy the stock from the shop, getting the items, they can then leave the GUI, and Player A has the stock, and Player B has the duplicated stock.

Hope this will be fixed ASAP, as this this most critical issue.