There are several concepts of duality in mathematics. There is the duality in the sense of category theory, and there is duality in optimization, and there is duality in order theory (which is closer to the one in category theory, I believe).
I don't know too much about these topics, so I can't fully judge that myself. But the tag duality-theorems seems to have questions from all sort of dualities in mathematics. That seems like a particularly bad practice in my eyes, at least if those concepts are "disjoint" or "orthogonal" in some sense.
To drive the point home, consider the tag "regularity" in which every question about objects which can be called "regular" fits. That's an extremely bad tagging philosophy.
Since I don't know too much about duality, I wanted to raise this on meta so someone else could perhaps clarify things and write tag wikis, and then we can decide whether or not this tag needs a thorough cleanup, or maybe removal.