How to reproduce this bug:
1. Put an Item in the shield-slot and take one item in your right hand
2. open your inventory and use a number on the keyboard to place another item directly in your currently selected slot
3. Press the F Key to switch the items between your hands
=> now you have duplicated the item you had in your right hand before step 2

Edit:
This bug seems to work only in creative mode.

I hope you understand my description!
Greetings from germany