The bug
Maps can be duplicated by taking them out of the cartography table GUI by using the hotbar keyboard shortcuts.
To reproduce
Create a new map
Put it into a cartography table
Put an empty map into the second slot
Hover with the mouse over the slot with the map copies
Press number keys to put the maps into your hotbar slots
→ ❌ The maps from the cartography table input slots are not removed, thus duplicating the maps
Apparently, this was only fixed for shift clicking (MC-148185)