Not sure if this is a bug, or if I'm just missing the logic here:

Create 3 boxes - boxes 1, 2, and 3.
Select them and Group.
Click on box 1 to select that individual box
Drag copy or use the copy tool to create a copy of box 1 = box 4.
Deselect

Selecting in the Objects browser now shows:

The first group I created - originally containing boxes 1, 2 & 3 - now contain boxes 2, 3 & 4.
A new group containing just box 3.