"Where there is light, there is a shadow"
There are two sides to a coin, two faces to a person and always two choices to life.
Growing up we all must've faced the challenge of choosing between two things, the greater good or the lesser evil. Eating either pizza for dinner or a burger; studying either HL Math or HL Physics. These were simple forms of duality that were embedded in our daily life.
How else does duality exist in life?
Imagine that you live in Tijuana (Mexico). There is a powerful mob leader who kills innocent people in masses, takes over businesses he likes and ruins the ones he doesn't. However he is a loving husband and father who keeps his family safe from all the danger in the world. One day another mob catches him without his gang and murders him. The city celebrates as now he will not terrorise them anymore. In this given scenario there are lot of contrasting examples of duality. The harsh reality of the many opposites; the powerful and weak, the harsh leader and loving family man, life and death. While the locals would cheer for the deceased mob leader, his family would mourn his slaughter. The foolish locals would think they're free of trouble while forgetting the emerging gangs that will now take over now that the main gang has lost their leader. Every decision has its consequences that impact different people differently, either positively or negatively.
Let's think of duality in humans. As Robert Louis Stevenson portrayed in his book 'Dr Jekyll and Mr Hyde' there is good and evil within every human, what matters is how we act make decisions that portrays which one of them we represent as a person. That side has been covered vastly, so I'd prefer to elaborate on another kind of human duality. Two facedness.
We've probably seen people acting differently around different individuals and we most likely do the same too. There could be several different reasons for this. One could be the genuine belief that your personality isn't enough to be liked by someone, or maybe you're just not comfortable enough to show them all of you. It could be because you don't like them and you don't want them to know, or because you like them and you don't want them to know.
Duality exists in several different ways in life. It is important, inevitable to come across and better to come in contact with in life.