WordNet sense 1 (a house with two units sharing a common wall):

duplex house, semidetached house

WordNet sense 2 (an apartment having rooms on two floors that are connected by a staircase):

duplex apartment

WordNet sense 3 (having or involving or consisting of more than one part or entity or individual):

multiple

WordNet sense 4 (reactive or functioning or allowing movement in two usually opposite directions):

bidirectional