When I read the following words of the Buddha, I see Nirvana as perfect non-duality:
[Aggivessana Vacchagotta:] “But, Master Gotama, the monk whose mind is thus released: Where does he reappear?”
[The Buddha:] “‘Reappear,’ Vaccha, doesn’t apply.”
“In that case, Master Gotama, he does not reappear.”
“‘Does not reappear,’ Vaccha, doesn’t apply.”
“…both does & does not reappear.”
“…neither does nor does not reappear.”
"How is it, Master Gotama, when Master Gotama is asked if the monk reappears… does not reappear… both does & does not reappear… neither does nor does not reappear, he says, ‘…doesn’t apply’ in each case…
Freed from the classification of form, Vaccha, the Tathagata is deep, boundless, hard to fathom, like the sea. ‘Reappears’ doesn’t apply. ‘Does not reappear’ doesn’t apply. ‘Both does & does not reappear’ doesn’t apply. ‘Neither reappears nor does not reappear’ doesn’t apply.
Please compare the above words of the Buddha to the Non-Duality Chapter of the Vimalakirti Sutra:
In the words of Rev. Walpola Rahula, “Nirvana is beyond all terms of duality and relativity. It is therefore beyond our conceptions of good and evil, right and wrong, existence and non-existence.”
Parinirvana is a perfect state of non-duality, beyond existence and non-existence, beyond any sense of separation or distinction between Nirvana and oneself.