I made a simple house which I had intended to group and copy a few times, and was made of some individual items and some groups (the windows and porch were groups); if I had to guess it was made of maybe around 100 items. But, when I box selected it and combined it all into one group, the gizmo menu stated that it was actually made of 5,104 items.
I assumed the counter was just bugged and displayed the wrong number, but no, when duplicating the house it duplicated some of the original groups 3 to almost 60 times. The porch in particular was duplicated the most.
I don’t know if there’s a pattern as to how items get duplicated however many times, but I think the game might have tried to use every single one of each item in the group that I owned; I had probably over 2,000 canvas cubes not placed down in my inventory and after deleting a few cubes in the duplicate (each copied almost 60 times) I noticed I had only 350 not placed down, so I can only assume it used up all of them.
Exiting the condo and re-entering seems to have fixed the issue with the initial group being made of 5,000 items; the item count is at 150 now.
Notes / Media
The house I duplicated, and the 5,104 items in the item counter:
Pain: