Dropping items in creative mode, by "Ctrl - dropkey" dropping, in the survival inventory view will come back into the inventory when you open another container. This dupes the items since they are still dropped on the ground, but also re-appeared in the inventory.

How to repicate:
1. Go into creative mode
2. Get some items in the "Survival Inventory" of creative mode.
3. Drop items out of the "Survival Inventory" by hovering over the itemstack and pressing the "DropKey" of "Ctrl + DropKey" to drop the full stack at once.
4. Close inventory and open another container like a chest or craftingbanch
5. Items are back into your inventory and are also laying on the ground.