Yes it is always one cube, just that it is located in two collections. It’s just like having one kid and two rooms, he can be in one room (collection one) or he can go to the other room (collection two). Just that in Blender it happens that the kid can be in room one and two at the same time. Your cube can live in both collections at once.
This is handy when you have to do some scene organization and you need one asset to be in more than one collection because you have to keep turning one collection on and off but you don’t want that object to be hidden, or something like that. There’s lots of situations.
