Searched for this bug in the unresolved bugs (via Excel export): no similar error found.
Bug description: It is possible to duplicate items in the inventory by following these steps:
1) Open the inventory tab and select the item that you want to duplicate, and put it in your "hand" inventory (i.e. one of the 9 slots).
2) Switch to the "Survival Inventory" by mouseclick
3) place the mouse cursor over the item (mouse hover) in the "hand inventory" that is to be duplicated. It is not necessary to hold the item in the hand
4) press q to drop the item while the mouse hovers over it
5) if the item is not picked up automatically, close the inventory and move to the item
6) you will hold two items in the inventory.
Many thanks to my daughter Ronja (10y) who reported this bug to me. As she is still learning English in school, I forward you this bug report on her behalf. All kudos belong to her. 🙂
Confirmed, but, given this is only in creative mode (Tried it in survival with no such duplication), and you can get as many items as you want in creative anyway, I don't see a big priority for this one.