First of all: what is duality? Duality is the denial of the inner unity of the universe. It is the belief in the division between people, and man versus nature; men against women, races against each other, nations at war. Duality is a mental illusion that imprisons humanity; an illusion that arises when our thinking is no longer based on love, but on fear.
Atlantean people were more cosmically oriented, they were aware of their origins. They were more mentally, more intellectually, developed and they had the powers of the third eye. They also looked a bit different, they were taller and lighter in color, and they considered themselves superior.
The consequence of their incarnation on Earth was that a class society emerged in which the Atlanteans formed the upper class, the ruling class. Among other things, they had come here to help and protect the Earth people. But they also looked down on those earthlings, considering them less evolved beings.
You can compare what happened then to what happened much later in human history – inspired by subconscious memories of Atlantis – during the Age of Colonization. The white colonizers saw themselves as those who came to enlighten, Christianize, and instill modern civilization in the “savages”. They even saw this assignment as “the white man’s burden”: it was the white man’s duty to enlighten the rest of the world.
Then, in subsequent incarnations, the Atlanteans mixed with the Earth people. They became part of them. However, the long-lasting Atlantean civilization, which lasted about a hundred thousand years, did have a deep psychological impact. It functioned as a kind of energetic mold for future societies.
Earth people began to imitate the culture of their previous overlords, trying to recreate Atlantis in their way of life and work. And with that, the mistakes of Atlantis were also repeated. They had internalized the dualistic ideas that arose at the time of Atlantis.