Every piece of knowledge must have a single, unambiguous, authoritative representation within a system. The alternative is to have the same thing expressed in two or more places. If you change one, you have to remember to change the others… It isn’t a question of whether you’ll remember: it’s a question of when you will forget.
– A. Hunt and D. Thomas on the “DRY principle” (don’t repeat yourself) from The Pragmatic Programmer: From Journeyman to Master
I might add that the same applies for documentation! – JB