Duality is an either–or perspective.
Duality is the bastard child of polarity gone wrong.
All of existence has polarity. This is known as Law of Polarity.
At a psychological level, duality is when, for whatever reason, we devolve into a singular, uni-polar perspective that rejects other aspects of reality. Duality-thinking is often made of beliefs, which are often built on incomplete polarity charges.
At a third dimensional level, physical life naturally has duality built into it (which is really just the extreme densification of the greater, harmonistic Law of Polarity). There is night and day, man and woman, fire and water, etc.