Non-duality in Buddhism is the Mahayana doctrine. Nagarjuna refutes all positive metaphysical theories and views leaving standing only the neutral or 'middle way' theory required for a doctrine of Unity. This denies the fundamentals status of all distinctions and divisions.
Thus Nagarjuna's proof is able to serve as the philosophical foundation of Middle Way Buddhism, Taoism, advaita Vedanta, Sufism and Christian mysticism as presented by Eckhart, Nicolas de Cusa and their like, and by modern books by authors such as Paul Ferrini, Keith Ward and David Bentley Hart. It is the acknowledged foundation for the monumental exposition of Christian teachings in 'A Course in Miracles' and 'A Course in Love'.
Nagarjuna is thought to have constructed his proof in order to address the divergence of views among the sangha of his time. He couldn't have done a more thorough job, but somehow his efforts didn't work. Yet his proof is unassailable and nobody has ever shown that his result is incorrect. He logically proves the non-dual nature of Reality and there is no reason every Buddhist should not accept this proof.
The topic is something of a can of worms since Nagarjuna's logical argument was designed to persuade everybody to agree on a single fundamental doctrine, and naturally this requires that many people change their views. Generally speaking, looking beyond Buddhism to 'mysticism' as a whole, it seems there is a growing consensus and understanding that the principle of non-duality is required for the knowledge claims of the mystics, such that if Reality is not the Unity of which they speak then their self-avowed knowledge would be impossible.
To confirm the endorsement of non-dualism by mystics across the ages is quite easy and it is actually difficult to miss once we know what to look for. But it is a difficult idea and so it takes time to learn what to look for. Thus it is often missed, It is invariably missed by academic philosophers, who rarely know anything of it.
It's a pity we cannot all agree on this since a denial of non-duality weakens the philosophical and intellectual plausibility of the Buddha's teachings and, according to Nagarjuna, renders them incoherent.