The bug

Locking maps using the cartography table duplicates them if there is more than one map in the input slot.

How to reproduce

1. Create a map
2. Clone it
3. Place the two maps and a glass pane in the cartography table
4. Take the locked map (left click) and place it in your inventory
5. Pick up the locked map again
   → You hold 2 instead of 1 locked map