Making a new map while holding one in the offhand will duplicate that map

The bug

When holding a filled map in your hand and a empty one in your right hand, you duplicate the one on the left. This only works in creative. mode

How to reproduce

1. Take a empty map
2. Fill it in and put it in your offhand
3. Make the second map
   → It should now have gotten made 2 maps, one which is the offhand map, and the new map if you are in creative