Something I've come across in my studies is the explicit use of duality in the fiction I'm reading. Characters are put in direct contrast to each other in order to make a point as well as to further the plot. The novel I'm reading at the moment (a little-know story by Edmund Yates, The Silent Witness) initiated the introduction of the two female characters by contrasting them in every way. One was fair haired, the other dark. One was a prattler, the other was full of sense. It may be wise to point out that the dark-haired intellectual quickly lost those traits when she was separated from the woman she was deliberately set up as opposite to.
Duality was a common tool of Victorian writers. It helped inform the reader using various simple distinctions: fair/dark, bright/dim, strong/weak and, of course, that old favourite pretty/ugly. It was used to great effect in Victorian fiction; think of Marian/Laura in The Woman in White or Mina/Lucy in Dracula.
It's an effective tool for any writer really and one that's still evident in every walk of fiction today. My favourite novel, Sarah Water's Fingersmith sets up a contrast between Sue and Maud from the beginning. The Harry Potter books are especially verbose in this kind of method: think Harry/Draco, Harry/Cedric, Hermione/Pansy, Neville/just about anyone.
It's a simple way of differentiating between characters. The idea of hair colour is one that seeps into even the best writer's conscious: if A has blonde hair then let's make B's dark so we can spot the difference. It's a handy marker but there are several pitfalls.
Firstly, if you create a character in response to another then the second character won't be as rounded and three-dimensional as the first. Not if all you've done is create an opposite for your protagonist. Some Victorian novels, especially sensational ones, suffer with this. The title character of Lady Audley's Secret is evil, for want of a better term. Her opposite in the novel is arguably Clara Talboys who, apart from having a desperate desire to find her missing brother, is an ineffectual human being. This had the desired effect of making the audience root for Lady Audley instead of Clara but it didn't do much for the depth of the novel.
A second problem with duality is that it can hinder character progression. If, as a writer, you're so intent on maintaining a distinction between various character then you can ignore the potential (or in some cases, necessity) for character evolution and change. This is extremely evident in serial television more than anything else. This is how the villain keeps going for years on end without being touched in any way by their deeds: think Tracy Barlow in Coronation Street, Don Beech in The Bill or Janine Butcher in Eastenders. What these characters loop back to is a desire to cause trouble, whatever brief epiphanies they may experience. They are there as the token villain and heaven-forbid they turn over a new leaf.
Finally, I'd say that duality can quickly become predictable and boring if not in the right hands. Some descriptions, especially in the Victorian books I'm reading at the moment, are cringeworthy in their attempts to set up a distinction between two characters.
The thing about contrast is that it should work on a primarily subliminal platform. If you have to point it out then you're not doing your job properly.