Instruction file imported from iPzard/coding-agents (.cursor/rules/clean-code/duplication.mdc). Copyright stays with the author.

Duplication (DRY)

• Don't Repeat Yourself. Every piece of knowledge must have a single, unambiguous, authoritative representation in the system. Same code in two places is one of the strongest smells.
• Diagnose the kind of duplication before fixing:
  • Forced by language or framework boilerplate — accept it, or use a generator/macro/template to derive it from one source.
  • Same knowledge stored in two places (e.g., a length field stored alongside start/end indices) — compute one from the other, don't store both.
  • Copy-pasted because typing the same logic fresh was faster — extract it into a shared function.
  • Different teams independently reinventing the same wheel — centralize the canonical version and make it discoverable.
• Fix it: Extract Method (same code in two methods of one class), Pull Up Method (sibling subclasses), Extract Class (unrelated classes), Substitute Algorithm (different algorithms doing the same thing).
• Caveat: when the duplicated chunk uses many surrounding local variables, extracting it produces a long, awkward parameter list. Confirm the extracted function's signature stays reasonable before committing to the refactor — sometimes the duplication is the lesser evil.