1. Go into creative
2. Press Q while hovering over an item (stack)
3. Pickup the dropped item.
4. You picked it up twice. In case of unstackable items, one instance will be invisible until the slot is updated.
this only works in creative (asfas I know).
this is also usable with ctrl+Q and will dupe the whole stack on pickup.
Won't dupe full stacks but will dupe a stack of 63 or 15 or freshly duped full stacks.