I think the items with hardness 10+ should stack enhanced digging mod
For example if I'm wearing a hand item and a neck item both give enhanced digging and break items which usually take 7 punches to break, I only get 1 enhanced mod effect
While if I was breaking for example steel block which takes 20 punches, a double enhanced mod will apply to make it breakable after 10 punches only
Or maybe a separated item idk