So basically, I was in the newest snapshot version just messing around, and I eventually began placing lingering potions in a chest. When I was copying them and shift clicking them in, I happened to press my drop key, which dropped the lingering potion onto the ground. I then walked towards it to pick it up and opened to chest before I picked it up. When I picked it up though, I felt that I had more lingering potions that I had before.

So I tried to see if my guess was correct. Basically, if you open your inventory and use to drop key to drop the item (I think it can be anything, whether stackable or not), immediately open a chest (or anything that you can interact with that opens your inventory), and you'll have an exact copy of what you just threw down.

Steps to Reproduce:
1. Throw item on ground (As far as I know, it can be anything)
2. Interact with anything that opens your inventory.
Bam! You have an exact copy of your item you just threw down. Now you can pick up the one you threw down!

Hope this helped!