Every single thing that exists on this earth exists with duality. This duality is almost every where. Short or tall, right or wrong, merit or sin, beauty or ugly, male or female, up or down, everything or nothing ....like wise this duality is endless.
But every where this duality is on a process to become one. That means duality is always trying to become the non-duality state. Non duality state happens with the right union.
If we look at this with the right vision, through out all spiritual processes that exist on this planet is talking about this same thing in different ways and languages. But actually the essence is... they all talk about the same thing. Which means the oneness, the non duality state. Every spiritual process is exploring this in numerous ways and gives us this message in a way that we can understand this.
Duality is the nature of existence. Nature cannot survive without this duality. In other words this can be understood with an example, if the sun does not rise then there is no light. If there is no light, there will be no life on earth. But for the light to exist the darkness has to be there. Every day the sun rises and also the sun sets. Like wise every process happen as a cycle. The duality is every where.
The humans has the capacity of coming to a state of non duality. Non-duality state does not create any cycle. There is no separate identity for such a person.
Our life on earth is continuing between this duality. If the life is like a river. Then this river is flowing between the river banks of happiness and sadness.
Every individual can decide which side they want to be. Whether to be happy or sad, whether to be in hell or in heaven.
Also there is another choice. You can stay in one of these sides in this duality or you can get out of this both and come to state of ultimate bliss - That is non dual state.