Items are duplicating (and the duplications turn into ghost items, which will disappear) when you press Q and F at the same time

When I press the F key (bound to "Swap Item With Off Hand") and the Q key (bound to "Drop Selected Item") at the same time, with at least three or four items in my main hand, it gets duplicated.

In Survival mode, the duplications are just ghost items, which means they get removed when you click on them or open up a crafting menu. In Creative mode, they stick with you and are treated like normal items. (If you try it, it may not work every time.)