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.
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FaRo1
@Decent60 Also in the hotbar, but you have to drop it from the open inventory, not by selecting the slot.
Can confirm for 16w14a, 1.9.2, 1.9.1, 1.9 (that's all I tested)
Steps to reproduce:
1) Enter /gamemode 1
2) Place item into inventory but not hotbar (doesn't matter how items gets into the inventory).
3) Press q and drop item.
4) Open up a container (before it gets picked up if you're that close).
5) Inventory will still have item and item will be floating on the ground (or inventory will show 2 of the same item if picked up near the same time as opening a container).
Doesn't work with Survival or Adventure mode
Doesn't work if you reopen inventory after pressing Q but before opening a container.