some thoughts
I'm writing this in the wee hours so I might have missed a few points.
== to the point version ==
What is duality? The prerequisites of duality requires a boundary to which one describe a binary-ness and a certain frame of reference. Like ying-yang, serial-parallel, digital-analog, male-female, etc Ask yourself what is the frame of reference, and what is the boundary of the item under consideration.
If you think of male and female, as one boundary (that which main stream society accepts), it's true. If you expand that boundary, it'll include variations like the other sexes. Like men with female organs, and the reverse. Or if you expand some more, men with dual organs, female with one of each, etc. labels of male-female start to break down and requires another perception like perhaps, percent deviation from average genetic structure.
so having beaten that horse... duality is just a term to point out that 2 different types are observed to have the least conflict and generally get along better than the other 15 permutations (or pairings). If you define the boundary of 2 types, and the frame of reference from socionics, then the definition of duality works. If you expand that boundary to holistic and encompassing idea of duality, well then we'll need much more reference books.
But for the purists, there is no natural language to describe psychological processes. Logic has their math, circuits have their networks, human communication has language. Invent a whole new language with weird symbols to describe psychology or cognitive processes and you might have something. But until that time, we're stuck (at least in here) with English.
== long winded version ==
The opportunity cost of labels or the act of labeling is to draw boundaries. These boundaries are necessary because we are trying to understand something in which is a different frame of reference. How do you describe mass to a point particle? How do you describe the color red to a blind person?
The boundaries can be strongly defined, weakly defined or relative or by means of some function, or whatever we need to help us to understand some part of it through our perceptions.
For non-omnipotent or non-omni-conscious humans like us, we make approximations on the universe to the extent that it is humanly possibly to understand. For the case of understand something within its natural element, thus not requiring labels, becomes difficult but it does exist. Sharing this knowledge requires the limitation of (besides human understanding) language or in general communication (with all the verbs, nouns, syntax, constructs necessary).
But were are experts in a few things that we generally assume. How do you explain what being human is to an alien who is considering a (genetic/surgical??) species change? What does it mean or feel like to be of a different sex? Through language and the perception of both parties, limitations exist, but it's better than nothing.
While some paradoxes are temporary, and others permanent (due to limitations of humans or whatever), the word itself presents a vantage point or a certain derivative of the truth or holistic nature. Yes, labels are labels, and be it relative, I bet it would be helpful to tell you that a hot looking person is staring at you in the bar. Would you contest the relativity of the label and state that it's not in true form? or would you take a look and go say hello? In short, labels helps the ball rolling a bit more.
Now for duality. To understand the relative label or term "duality" one must appreciate it's correct context. From MBTI or socionics, it's a theory, or an attempt to describe or emphasize (not define, although some would argue) a certain pairing of 2 types in the universe of MBTI. MBTI is an approximation of humans, not of animals, rocks or anything else (though it would be really fun to type your dog or other beasts)
From what I've read, dualities are not to infer contestation or conflict, but rather a statement of something that exists or humanly observed. A few sites use the label duality to mean slightly differently things. I just know from socionics that duality just means a pairing of 2 types that is observed to be the most comfortable, or favorable (depending on who you ask).
I being, INTP, and having met my dual ESFP, what is written is so far true. Does it mean that it is the correct description of that which exists. No, but it helps us from A to B.