What is the difference between contrast and duality?

In literature, these two terms, “contrast” and “duality,” are sometimes used to discuss pairs of utterances or styles or plot configurations.  “Duality,” as the derivation of the words implies, simply means “two” of something:  Football and tennis are two sports, for example, and may be practiced, followed, observed, etc. But “contrast” is a particular “duality,” one in which the pair of terms or utterances differ from each other in a way that highlights their difference:  Football is a contact sport, while tennis is not.  In literature, a pair of protagonists (say Macbeth and Lady Macbeth) is a duality; if we discuss their personalities, we may concentrate on the differences in their zeal (a “contrast” discussion) or just discuss their mutuality and similarities.  In poetry, we can pair up (a duality) Yeats and Frost;  we can “contrast” Yeats’ patriotic view with Frost’s non-political view, or we can simply discuss their mutual skill with pastoral descriptions.