Asserting the duality of human nature means asserting that human nature, despite its complexity, has two fundamental aspects.
Some people believe that every human being has a good side and an evil side. “The Strange Case of Dr. Jekyll and Mr. Hyde” is a classic story that illustrates and explains this view.
Not everyone believes that it is useful to divide human nature into two fundamental aspects. I don’t.
Some believe that every human being has a masculine side and a feminine side. I think that belief came out of the psychoanalytic tradition, via Jung. Personally, I think it would be simpler to conceive of a woman’s “masculine side” and a man’s “feminine side” as groups of psychological traits that happen to be common to both sexes.
To me, human nature seems too complicated to be reduced to a simple dichotomy.2