Duality is a slight of hand. It is a line drawn in the sand where none exists. It is the parsing of All That Is into distinct semantic thought-concepts that hide from the part its concurrent existence as the Whole. It is the subjective narrowing of attention upon an object, shrinking its full context, and ignoring the ground connecting it to the subject.
Reality unveils the connections we trick ourselves out of seeing. It makes a game of the lines we draw upon it. It reminds us that thought-concepts are only limitations of the thinker, not of the world through which our thoughts ripple like waves on the surface of the ocean. It pans back and shows us that everything we analyze in isolation are aspects of a greater context that includes the whole Universe, enveloped by and filled with infinite ground of Existence, and you are That.
Reality is the Garden of Eden. It is Brahman. It is Buddha-mind. It is the Uncarved Block, aka the Tao. It is the Light of Pure Awareness that shines in all things.
Duality is the result of taking the fruit of the Tree of the Knowledge of Good and Evil at birth, a symbol of the higher functions of semantic mapping in the human brain that create a fully subjective self-other dichotomy. We have all done this, and have thus all fallen short of the glory of seeing Reality as it is. It is the state of sinfulness. It is maya. It is dukkha. It is disharmony with Tao.
Non-dual thought, the result of contemplation into what is true and real as opposed to fleeting appearances, restores the direct perception of Reality. It brings a release from the illusion of thinking of oneself as a separate being in the universe, and a knowing of oneself as a unique manifestation of Existence itself, connected to all other manifestations through your co-existence as the Self. It is salvation. It is moksha. It is bodhi. It is the de of action in harmony with Tao. It is the end of seeing a subject-object dichotomy at the ground of your existence, and the beginning of a supersubjective sense of “I and I.”
This restored sense of Reality is what we discover intuitively after remembrance of the Self, returning to the state of existence we never truly left. It is Heaven. It is Satcitananda. It is Nirvana. It is Tao.
These are all fancy ways of saying one thing: Duality is a grand tour of all that appears to be. Reality is All That Is.
Enjoy the tour; never forget where it is leading you.
Two swirling opposites, and one whole unbroken circle –which do you see first?
Originally posted 4.17.14, edited 1.16.20
Not One, Not Two 
Very indeepth thinking and good summary, Thank you JP
Yes, the separation is a figment of imagination. No separation truly exists. Duality is an illusion. We are in nonduality and have always lived in nonduality.