What is the bug?
The bug is: item duplicates on the ground (check how to replicate) but and you still have the item (the duplicate is the one you have in your inventory), also if you open a gui (like a chest and not the inventory), the duplicate will disappear, if you open the inventory and you try to get the item, it will disappear, if you have a tool (like a pickaxe) you can mine the a block (after a few tries), if you try to drop the duplicate, it will disappear

How to replicate this bug:

1. Get two items (Random)
2. Put the first item in your offhand
3. Hold the second item
4. Press F and then very fast Q
Note: This doesn't work always