Duality is a way humans understand contraries or opposite energies or forces in their environment. Physically can be the contrast between cold and hot, light and dark, day and night. Emotionally we feel duality as the contrast between sadness and happiness, between crying and laughing. In a more complex level we can see duality in nature as the quality of creation and destruction. Moral duality is the one that difference between good and bad, and it is the kind of duality more often found in Christianity, but also in some forms of pagan beliefs. More witches and occults talk about white and black magic distinguishing the intention beneath the act or performance. This distinction although is rejected by many, is very often done on a practical level. Pagans who follow the solar wheel of the year, understand duality as something cyclical in nature between light and dark, but not necessary related to a moral or ethical value, but to different states of being of animals, plants and human behaviour during the forth season, winter can be understand as opposite to summer and autumn as opposite to spring. Moon phases have similar opposition between full moon and black moon. In Kabbalah, a Jewish mystical literature that highly influenced western traditions, there is a clear duality between the left and the right columned of the tree of life. This two opposite are severity or the desire to receive, and mercy or the desire to give, and usually there is a moral connotation in those opposites, they are both considered necessary to our evolution and the solution to that opposition is to consider the left contained in the right. Evil is not considered a quality of the left, but it is the quality of right within the left, and is the consequence of the fall. In some forms of paganism, the duality between male and female nature, not seen as opposite but from a separate spiritual nature is very important, as some define deities as haven a gender nature. Paganism often considers a monistic worldview where opposites are understand as part of a hole.
Sometimes duality is express in dualism in religion as the oppositions of two or more gods and demons, causing the world to exist, this is in contras with monotheism which consider one principle as the origin and often have many problems to explain the nature of evil. While most modern pagans finally used to consider all this different forces as part of one hole, this is not necessarily the ancient paganism view.
In ancient Mesopotamia we found the opposition between old deities concerning chaos: Apsu, the Abyss and Tiamat, the Sea dragon, and new ones bringing order: Marduk a kind of demiurge or creator. In Egypt there, there is not an explicit duality, but we found the contrast between deities as Seth and Osiris, the first a sterile god of the dessert, the second a fertility god related to the waters of the Nile. As for ancient Greece, different forms of explicit dualism existed between different schools. Empedocles considered a cyclical alternation within principles as Love and Discord. As many other current he considered the existences of daemons (souls) as divine beings that have fallen into this world and live in the ‘foreign robe of the flesh’ as animals, vegetables or human. This is the primitive accident further know as the fall. Orphism described a body-tomb concept, seeing the body as a prison for the soul, plus the idea that everything comes and return to the One, an opposition between the one and the many. For Plato there exist to irreducible principles: the idea and the chora (material vessel) and the world is conditioned by material needs, without the soul the world will be a kind of mechanic system. During the Hellenistic period rise a more extreme form of dualism including itself some concepts from Plato and Orphism, the Gnosticism a pagan-judeo-christian movement that considered matter as evil and spirit as good. They adopted the concept of pneuma, breath or soul, different from psyche, and divine in nature, that needed to be free from the material world of the demiurge, to attend the pleroma. Zoroastrians narrated the tale of the opposition between a benefice Ahura Mazda and a destructive Ahriman. The mysteric Mithraic religion was strongly influenced by persian dualism too. Mani who synthetises many believes from early Christianity, Judaism and gnosticism stand for polarities as light and darkness, good and evil, peace and war.
It is very difficult to stay where and when dualism as opposition of good and evil appeared in religion. Most Neopagans try to transcend this concept and follow a moral of harm no one, without having a set of rules or a fixed moral code.