Much has been said about the importance of reducing duplication in software. For example, J. B. Rainsberger has "minimizes duplication" as the second of his four "Elements of Simple Design", and lots of the teachings of the Agile community stress the importance of reducing duplication when refactoring code. Inspired by Kevlin Henney's tweet last week, where he laments that programmers trying to remove duplication often take it literally, I wanted to talk about the different kinds of duplication in software. I've just mentioned "literal" duplication, so let's start with that.