"Duality or difference (bheda) cannot be applied to the Supreme Lord,
because He is in full oneness with Consciousness."
This apparently difficult-to-understand statement is very simple in the end.
Consciousness is the state in which a conscious self lives, i.e. the reader (I am not speaking of anyone else ever). Consciousness is always one Reality and cannot be delimited by anything at all, because It is beyond space and time.
For example, if the reader says: "I am different from that person who is over there"; that is false. Why? Because his statement is based on forms (physical bodies), location in space, age of the forms, ideas, etc. So, the reader would be saying that another person is different from him because his body is not like the reader's body, or because the mind of the other person is not like the mind of the reader, etc. etc. But the reader would be forgetting something that is vital: "If he perceived all those differences between him and the other person, therefore all that being perceived (i.e. existing) is one with Consciousness". Whose Consciousness is it? It belongs to the one who is conscious, of course: the reader (again!).
All in all, he is conscious of all those differences, and therefore they are one with Consciousness (the state where the reader lives day and night). And if the reader says that all those differences are "different" from Consciousness, so how did he manage to be conscious of them? Why? Because those differences could not have been perceived by him at all in their being non-existent, i.e. he would not have been conscious of them all.
In short, there is always "one" Consciousness perceiving all the differences.
Although duality exists for everything in the sphere of space and time (objects, bodies, minds, etc.), as Consciousness is beyond space and time, duality cannot affect Consciousness ever.
Duality or difference can never be proved in the case of Consciousness because there is insufficiency of means (anupapatti). In other words, the Supreme Lord (YOU, reader), in His being one with Consciousness is always Eka or One (Solitary), and never two or more.
Another simple example: Look at the objects around you right now. All of them look different from each other, don't they? All those differences appear in space and time but not in Consciousness. Consciousness is the state of a conscious being, i.e. You. Despite your seeing different objects in space/time and duality is prevailing on that level, Consciousness is always the same, viz. It is not experiencing duality. There is not one Consciousness for this object and another Consciousness for another object, and so on. NO. It is only "one" for all of them.
If the objection about one Consciousness per person is raised, the answer is: "There is only one Consciousness". Whose is it? Yours! Consciousness of others, if outlined in thought, is always existing in your own Consciousness. You live in your own Consciousness and there is no other way to live for You. Try to live in another Consciousness and you will see that that is impossible.
If you understand this teaching completely, you achieve final liberation in a trice. Final liberation does NOT have to do with chanting mantra-s all the time, worshiping the deity day and night, crying for God, etc. NO, it has to do with receiving Grace of the Supreme Lord and being very intelligent, i.e. to have an intellect that is good at reasoning adequately from a spiritual viewpoint.
The Grace of the Supreme Lord is raining over you by my notes of explanation, be sure. The remaining factor is development of your own intelligence. If you can manage to develop it or if you already have it, final liberation is around the corner for you because "it is pretty obvious that You are the Supreme Lord".
Chanting mantra-s, worshiping the deity and similar practices are only to kill the time according to my own experience. Why? Because Consciousness is Self-existent. One does not have to practice anything to realize It really.
It is only his own foolishness which leads a person to think that Consciousness can be achieved by practice. Consciousness cannot be practiced, reader, because It is your essential nature. One does not need to practice to be what one is already. It is foolish to do so, from all the viewpoints. Imagine you saying all the time: "I am I, I am I, I am I". Absolutely foolish, right? People practicing for gaining Self-realization look the same in my eyes: "I am Consciousness, I am Consciousness, I am Consciousness"... yes, I already knew it... stop your inner parrot, please... thanks! 😄
Guru Gabriel Pradīpaka in The Paramārthasāra of Abhinavagupta