This has so far only worked in creative mode for me.
1. grab any item or block
2. open the inventory
3. pick your item/block and drop it outside of the inventory (won't work by pressing Q outside of inventory)
4. it will be picked up and placed in the inventory
5. drop it again
6. you will now have 2x items/blocks
7. repeat step 5.

If you already have more than one item this will also happen:
1. right click the items and drop it like previously.
2. it will stack with the others
3. right click again and drop
4. it will duplicate and stack

If you have many items in a stack something even weirder will happen
https://gifs.com/gif/y0rzJg
https://gifs.com/gif/vqQ89V
https://gifs.com/gif/mP7L3q

full video: https://www.youtube.com/watch?v=tkOi96yU6Xs