There are multiple meanings to the Duality of Man.
The Duality of Man can be the good and the bad that lives inside one single person, not one person only hosting one of them.
It can also be the Duality of the mind and the matter, or more spiritually, the physical and the spiritual, two states in which all humans exist.
Some famous philosophers even view it as the difference between your consciousness and your perception.
The mind is often the common factor when talking about the Duality of Man.
The well known greek philosopher Plato spoke about the Duality of Man when it comes to mind and matter, meaning that one part of man lives in his mind, thinking and observing, while another part moves and creates.
Later philosophers and thinkers have derived their own opinions about the topic.
The term is seen everywhere in history and modern times, especially entertainment.
Many of Shakespeare’s plays involve the Duality of Man, the thinking inside and the acting outside.
Modern superhero movies and childrens stories often have a very black and white perception of good and bad, which opposes the Duality of man, claiming some are purely good, and some just evil.