There is a philosophy that each person is born with a duality.
Gemini is the ‘twins of the zodiac.’ That itself kind of captures the
whole concept of duality, but on a philosophical level; you
have yin-yang, positive and negative, day and night, and left and
right. These are principles that we created. We made words for those
things to help us understand the world that we live in. Bringing those
two sides together, everybody has good and bad days, good things and bad
things, and even we have good shows and bad shows. It’s just trying to
balance those two things to make your life more fulfilling and trying to
make it the most positive experience you can.
Each person could be said to contain two individual characters.
There is the conscious self as opposed to the unconscious self. In the light of day we have a public self, while at night we can have a private, dark self where societal rules do not apply.
There is the good behavioral person as opposed to the evil persona. Each person has within themselves to do evil. It is how we check this bad twin is what keeps normal people from becoming criminals.
We have split personalities of the work ethic and the procrastinator, where there is a moving slide mixing the two elements to determine the course of our daily lives. Some days we are work horses, other days we are lazy bums.
We also have various emotional states that are constantly in flux. Love-hate relationships. Kindness versus cruelty. Extroverted energy to introverted paralysis. There are various shades of the emotional self that has many variables based upon cultural and environmental cues.
LOST had various themes which included a duality principle.
The mirror is a reflection of one's self. It also could be considered to represent the other side of your personality (usually a darker one).
Duality is a central principle in Egyptian burial mythology, where a person's soul is divided into parts which are reunited if the person passes final judgment.
Several main characters went through drastic phases in their lives. For example, Ben was a shy, introverted school boy who deep inside hated his father to the point of being an evil mass murderer and dark tyrant. But even then, the story twists unbelievably to Ben as a sympathetic nice guy waiting to cure his sins in the sideways purgatory. Fans were drawn quickly to the evil Ben more so than the redeemed Ben.
And the fact that LOST was set in two different dimensions is still a cause of great concern. Was the sideways world purgatory, heaven, dream state or alternative universe? Likewise, was the island time line real, imagined, science fiction worm hole or psychotic?
The series raised duality concepts but failed to clearly address them in a coherent fashion.