It means “something related to number two”. Usually relates to some two things coexisting in some conflict. It is hard to be more precise without a context. So some examples:
Wave-corpuscular duality means that in the quantum world things seem to behave in two different ways — both as discreet particles and as “waves of probability”.
The song “Duality” of Slipknot seems to be about some internal conflict of the human nature. The lyrical subject believes that there is some conflict inside them that can be fixed by either ”putting back together” or “separating the skin from bone”.
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