The Origin of Nonduality.
 

The manifestation is an idea postulated by Advaita that the many things appear, including us, but in reality are One. This idea only arises in the manifestation for there is nowhere else for anything to appear. Whether there is a reason for the manifestation is not addressed by Advaita but the idea that All is One is postulated as the only reality, so realization of this is regarded as Enlightenment.

Whether there is an earlier origin others may know more. It may be it was just made up by some priest in an attempt to obtain status and wealth. In any event it has endured and become the many faces of nonduality, most notable of which is the difference between the requirement that the nondual idea must be realized, and that it need not because no increase in connection to Oneness is achieved by realization as it is already Oneness not realizing.

Yes that's a description of the traditional nondual view that nonduality needs to be realized/experienced to be regarded as Enlightened. The other view, often called Neo Advaita, is that realizetion/experience will not increase the total connection that already is. The former has something to be achieved and will be open to some but not all. The latter has nothing to be achieved and therefore applies to all, no matter what state the seeker is already in, and is therefore totally trancendental. The dispute between the two versions has raged for decades now. For me there is room for both depending on the characters (Ego's) of the seekers involved. Traditional will suit some and Neo will suit others.

importance of realization
 

Realization is key in spirituality else its either toddler's ignorance (like people with total ignorance of spirituality) or net practice in sports under limited safe conditions( seeker trying to appreciate & understand spirituality genuinely ) or pretence (wilful deceit of realization when there is none) a show biz for material considerations . Neo Advaita is the current de-facto default status and there is nothing to be done .You lead an ordinary life without any need for development.

Realization is where non-duality has to work in realtime instantaneously spontaneously and the spirit in such case comes and becomes the player himself in the life such person. In such an intense life supersoul himself comes , witnesses , partakes and plays and enjoys the seeker's life . And that's how such people become God , Son of God or Messenger of God . Geeta verse may be of relevence here.

Bhagavad Gita has description of Sthitapragna who leads such non-dual life . Adi Shankaracharya was the propounder of this when buddhism was born in India and he re-established it (Hinduism ) again in India within a short life span of 32 years single handedly roaming entire Indian sub-continent without any means of transport around 1000 years back. Vallabhacharya and Ramanujacharya also propounded with minor variants after some centuries when Mughals had invaded India.

There is no way for us to know the quality of the internal life of another, whether that be a supporter of Traditional or Neo Advaita. Either may be Enlightened and the other not.

There is some truth in this. And yet those teachers who come under the popular heading of Neo-Advaitism all seem to have had a moment of realisation. The total connection may have always been there, but these teachers all describe their particular experiences of realising this to be so. They may then teach that there is nothing to be achieved, but they seem to ignore the fact that they only know this because of their own achievement.

But what does achievement actually mean? It is not a case of attaining something new which was not already present. Achievement is more of a letting go and seeing what has always been present. And this (to me) applies to both traditional Advaitism and Neo-Advaitism.

Peace

life in duality
 

It's not that life in duality (in traditional sense) is bad. That life to can be great and there can classification/categories for the same like philanthropic billionaire/rich people , benevolent ruler/politician , righteus judge ,service oriented bureaucrats , good teacher ,innovative yet honest/ disciplined & productive workmen , saints , rishis ,scientists etc .

Though all such people may not be experiencing non-duality in strictest sense of the term , yet they have very high spiritual value and are very much respected people .

There is no way of knowing whether somebody else is enlightened, including the people you mention.

Yes some Teachers claim a realization event then say it is not necessary:). Whether it is or not can be judged by the seeker who can then select what suits them. In that choice there is a diffeence between Traditional and Neo Advaita in that the former insists that realization is necessary.

learning spirituality
 

Learning spirituality with modesty n honesty is the only way one can genuinely try to assess about the enlightenment of others. Otherwise superficially its just not possible to assess anybody.

We can not have binary categories like enlightened and unenlightened . People come in various combination shades like any color screen pixel could be a combination of RGB (Red green blue with each having 0 to 65k values) . Similarly people to come in various shades with 3 basic orientation
1. Spirit oriented activity - (sattvic)
2. Material oriented activity - (Rajasi)
3. Material oriented inactivity- (Tamasi) .

Every individual is made of these 3 orientation in varying degrees . Different orientation become dominant different time /place and situation . The one's I described in my examples are having spiritual orientation (sattvic) more than other 2 . The degree of such orientations define the type of the person he/she is .