Though at least in Berlin it's pretty common for multi-family houses to have a separate wing (Seitenflügel) or rear house (Hinterhaus) that are reached by entering the street door of the front house (Vorderhaus) and then exiting through a door behind the staircase into a courtyard before entering the second building, and at least in some cases each building has its own set of mailboxes, all with the same address.
I regularly have the problem that deliverers don't read my delivery note and don't listen to what I say on the intercom, and go all the way to the top of the front house before realising I'm in a different building altogether.