Switching to a hotbar slot with an item while pressing the drop key at the same time will cause the item to disappear. Switching from a hotbar slot will instead cause the item to drop, but also remain as a phantom item in your hotbar.

This appears to be a visual bug, not a duplication glitch, as updating your inventory GUI or interacting with the item's slot will fix the issue. That said, it does appear able to delete/duplicate items when performed in creative mode.

Steps to Reproduce:

1. Have an item in your hotbar
2. Switch to that item from a different hotbar slot with the number keys while pressing the drop key (q) at the same time.
3. Item disappears from hotbar without dropping. Update the slot or Inventory GUI to return it to inventory.
4. While the item is selected in your hotbar, switch to a different hotbar slot with the number keys while pressing the drop key (q) at the same time.
5. Item drops while remaining in hotbar. Update the slot or Inventory GUI to remove the phantom item.

example1.mp4

^example2.mp4

^example3.mp4