I would say that maisonette most usually refers to a residence within a larger building which extends over two or more floors, although it can also refer to what is effectively a single-level flat on an upper floor with its own private stairway and entrance door to the outside world (as opposed to having communal hallways and external doors). It also implies compact accommodation, not surprisingly given that the term is the French diminutive for little house.
Some maisonettes were converted from large Victorian houses, but there were also a lot of purpose-built blocks in the 1960s which were designed and marketed as maisonettes from the outset. It's probably one of those descriptions where you need to ask questions to be sure of exactly what the person using it means.
I've never heard of Vicky's British definition of a duplex before. I've only ever known it in the American sense, i.e. what would be called a pair of semi-detached houses in the U.K.
