Most programming courses teach it as an afterthought, if at all, which is strange because it motivates so much of the structure of programming. I mean, the evolution of computer languages has mostly been about finding higher and higher-level ways to eliminate duplication in code, and many language features are all about ways to remove duplication.

I certainly try to make this point in class. "When you find yourself writing the same thing over and over, you're doing something wrong." I say this in introducing variables, again in introducing functions, again in introducing higher-order functions, again in introducing inheritance....