A bug can appear when there is latency which makes it possible to duplicate the items.

In multiplayer, you must quickly equip a weapon which replaces the one you currently hold, then quickly put the unequipped objects in the trunk and return to the main menu. Sometimes you will still have the items in your hand on the menu. And when you return to the game, you will have them in the stash.

This does not happen often. I feel like it can only do that when the server is lagging.