I don't get it

Maybe I wasn't paying enough attention, but what was the setup of the duplex? Isn't a duplex in NYC a 2-story apartment? Like a duplex over a single is 2 floors over a ground floor rental. So they didn't actually buy a duplex apartment? They bought two apartments, one on each floor, with the old lady living upstairs? What about the rest of the building? It's obviously a three story, it would have a top floor unit and garden unit too, assuming they live on the first floor and the old lady lives on the second. Unless the other units just happen to both be vacant, but that's silly. It should have just been written so they bought the whole building and then changed the name of the movie to Rent Control or something.

You know, I don't find this stuff amusing anymore...