Probably a duplicate, but I didn't find one for 1.9.2 or 16w14a.
If you drop something from the player's inventory (not by selecting in the hotbar) and then open an external inventory (chest, hopper, enderchest, ...), the items are back in the inventory. Picking them up still works, so the items are duplicated.
I can give more details (videos, logs, etc.), but it's probably a duplicate anyway, so I'll first wait for a mod.