Since wordiness is not a quality of duality, I'll be brief.
The Spanish Royal Academy Dictionary says:
DUALITY
1. (from the Latin: dualitas): Existence of two different characters or phenomena in the same person, or state of things.
2. (Chemistry): Ability of some bodies to crystallize, under circumstances, in two different geometrical figures.
Now the dictionary has spoken, I'd rather let the paintings talk for themselves: the thin man always within the fat one (as Cyril Connolly has wisely pointed out), the man and the woman, different within the same dance, double flowers, or the chicken and egg paradox; not forgetting left hands which cannot be transplanted as right ones, or vice-versa.
Kinrocs