Suppose you're working on a room containing lots of objects, and you need an exact copy that is mirrored. This can easily be performed by selecting all the objects, then duplicating and mirroring them with Shift D and Ctrl M.
In this case, any time I make a change to this room, I would need to delete the duplicate room and contents, then perform the Shift D and Ctrl M procedure again to ensure that the room and all its contents are the same.
To make things faster and more convenient, is there a way to automatically do this? What I'm looking for here is something similar to the mirror modifier, where changes to the original piece are mirrored at the exact same time, so that you don't have to do it yourself. Unfortunately, the mirror modifier only works on individual objects, and thus, a room with hundreds of items in it would be a nightmare to mirror individually.