Wisdom is meaningful only so long as it bases on only un-contradictable truth, in the sense that it must not be contradictable. The truth must be such that it must be accepted universal truth.
In other words, non duality must not be opposed to the presence of duality. If there is such an opposition, non-duality would become the dualistic opposite of duality - when duality comes, non-duality goes and vice-versa.
Where there is duality, there would be no non duality and vice-versa. Thus, being limited by the presence of duality, it would effectively become a kind of duality in its own right - an oxymoron.
Therefore, non duality in the spiritualistic sense implies freedom from (or transcendence of) duality rather than absence of duality. Non dual wisdom implies "freedom from ignorance" rather than "absence of ignorance" - the freedom from ignorance must necessarily include the freedom to take on any and all ignorance.
Another way to understand this is that absence of dualistic attributes implies the presence of its dualistic opposite. True freedom from attributes requires freedom from both the opposing attributes, which effectively implies the freedom to include both the opposing attributes. Only such an attribute can imply true non-duality. Only when one becomes aware of the fact that the attributes of duality is dependent on the non duality and non duality is independent and can remain with or without the dualistic attributes then he will be able to judge what is permanent and what impermanent, and what is real and what is unreal.
Similarly, one may consider the usage of the word non-duality. Nonduality is not opposed to duality, but inheres in and through duality. To understand non duality, seeker must recognize that the whole experience of duality [waking/dream] and make the formless witness [non-duality] itself an opposite of duality leads to freedom from experiencing the duality as reality.
The sky, the star, planet ,sun , the moon, the earth, the mountain , the ocean and the person are all Ataman/soul, but Ataman/soul is not specifically any of these while simultaneously being all of these. This is because while Ataman is free from the sky, the star, planet, sun, the moon, the earth, the mountain, the ocean and the person , etc it is the formless substance and witness of all these.
