When we wrote code before large language models could write it for us, code duplication was rarely our problem, at least not if we worked alone or in a small team.
That has changed.
A principle born in a quieter time
"Don't Repeat Yourself" was coined by Andy Hunt and Dave Thomas in The Pragmatic Programmer back in 1999, a year when our biggest worry was whether the clocks would survive January. The idea is reassuringly simple. Every piece of knowledge in your system should have a single, unambiguous home. Write it once, point at it from everywhere else, go to lunch.
For those of us working alone or in small teams, DRY was almost automatic. You held the whole codebase in your head, all of it, including the parts you were ashamed of. When you needed to format a date, you remembered you already had a formatDate helper, because you had written it last Tuesday and were still slightly proud of it. Duplication was something that happened to other people, specifically large organizations where three teams could write three subtly different currency formatters and never once meet in a corridor.
