What does duality really mean? It means "manyness", many separate and independent "things". So duality means "of things" or "thingness." Our normal experience of reality is that of "thingness". To be dual is to be more than one - so we're really talking about more than one "thing". Yes?
What does this mean? It means that this world is made up of separate things, each standing alone or apart, independent in their existence. This paradigm requires the subject/object equation - an "I", a separate "seer" apart from the "seen" or objective.
Vedanta simply says that the nature of existence isn't separated, that although there is an appearance of "things", those things don't have independent existence. Existence is existence - it's not separated. The appearances are of that singular existence, although there is the appearance of separation. The appearance of thingness is called "Maya".
So let's reverse-engineer duality. Let's find out how this idea of duality or "thingness" is created.
Can you think of nonduality? Can you think of "oneness"? No. Because to say "I am thinking of nonduality" immediately requires a separate "you" and an object of thought - two "things".
What is right in front of you? A computer screen. To call it a computer screen is to create duality. Calling it a computer screen assumes the one who is seeing it, the room in which it is located, the space in which the room is located, the universe itself.
What are you sitting on? A chair? Calling it a chair immediately calls into existence (in concepts) the entire universe. All that is not-chair. Do you follow?
Therefore it is mind and language (concepts) which creates this duality.
What IS is what? Right here and now. THIS is what IS. Isn't THIS reality? Right here and now? Whatever THIS may be called or conceptualized. And it's ok that the mind does this - that's it's job. It's normal. But in this conceptualization, the "world of separate things" is created. Even that's not a problem. But along with this, suffering comes, because the primary "thing" in this equation is "ME", the separate self, the one who is the self-reference AS a separate thing. This paradigm creates the very idea of isolation and limit.
Why? Because IF there is a world outside you, independent of you, then you are limited and isolated BY the world. You are a small and insignificant piece of the universe, which has a beginning and an eventual end.
This is not reality. You are not limited, you are not finite.
Watch how the mind automatically places "thingness" upon what IS, and in doing so, creates the world in imagination, automatically imagining a world. Therefore the first thought is automatically false. The first thought is automatically dualistic. That's just the way the mind works.
If we try to not think dualistically, we will always fail. To even think about nonduality requires a thinker, someone who will know nonduality or Oneness. Can there be someone apart from Oneness to think about it? To know it? To objectify it? No. Notice how the mind must place thingness upon that.
So there really isn't any problem, as long as we notice that this is what the mind is doing. In seeing through this mechanism, we find that what IS, still IS. Nothing has changed. But that mechanism of "thingness", that creation of "partness" is ONLY the work of the mind. In fact that dualistic mechanism is really very relative and arbitrary. It depends on the conditioning of the mind, what has been learned, etc. Reality cannot be relative and arbitrary.
So what IS remains what IS - only there is no way to talk about it, no way to think about it accurately, truthfully. The first thought is automatically false and only dualistic. Does this mean we stop thinking?
No. We just see the mind for what it is. Then what IS remains what IS, without the belief that it is made up of "things". And if it's not made up of "things", then what is it? What is reality, in direct experience, without this dualistic process?
Reality is just YOU. The "real" you is just THIS, just this moment, this "presence" - whatever IS, right here and now, is YOU. If this mental process of "thingness" were to fall away, would YOU fall away? Wouldn't you still know that you exist? Yes - because that knowledge of existence is the only knowledge which isn't conceptual - it's immediate and obvious without requiring a thought, without requiring a concept.
Therefore YOU ARE - that singular existence, that nondual reality, is YOU.