In the past, some of these buildings (originally one building with one occupant family) were "redeveloped" into a few units. Each individual unit owner was a joint tenant - they owned a percentage of the entire building rather than a separate and distinct title for each unit. This was always a messy way to hold title, so recently there has been a push to convert these types of buildings into condos.
I'm not sure what you mean by your question - but some of those properties have been converted into condos and town houses. As a duplex, it is owned by one person, or perhaps there is shared ownership, distinct from whether or not it is rented. As a condo conversion, that same duplex is now two separate properties with two separate owners, although with portions of the property shared.