Currently if you add something to a group and then duplicate it, that new object becomes part of the group. This is usually OK if it is something small, but if for example you create a wall consisting of a number of separate parts and group it so that it can be repositioned, then duplicate the group, the group now consists of the two walls and they can't be repositioned separately. It can be a bit of a pain as the only way around it seems to be to ungroup the entire thing and individually select all the components again.