duplex apartment vs apartment

duplex apartment and apartment both are nouns.

Word	Noun	Adjective	Verb	Adverb
duplex apartment	Yes	No	No	No
apartment	Yes	No	No	No

As nouns, apartment is a hypernym of duplex apartment; that is, apartment is a word with a broader meaning than duplex apartment:

duplex apartment: an apartment having rooms on two floors that are connected by a staircase
apartment: a suite of rooms usually on one floor of an apartment house

Other hypernyms of duplex apartment include flat.

duplex apartment (noun)	apartment (noun)
an apartment having rooms on two floors that are connected by a staircase	a suite of rooms usually on one floor of an apartment house