Answer:
According to Émile Durkheim, duality of Human Nature is the opposition of the two following concepts: Soul VS. Body. (From a religious point of view) The Body is egoist and the Soul is Reasonable. These two concepts are in opposition and makes us the contradictory beings we are.
This opposition gives us the need for spirituality in order to bind them. (Or at least make them coexist peacefully.)
Durkheim call the duality of human nature: Homo Duplex.
Answer #2: WAR.
yin yang
bad good
dark light
sad joy