DRY Often Makes Your Code Worse

A practical walkthrough demonstrating how blindly applying the DRY (Don't Repeat Yourself) principle can make code worse. Using email and username normalization as examples, it shows how merging similar-looking code into a single generic function with many flags creates unreadable, hard-to-maintain code. The better approach is to focus on duplicated knowledge (rules, business logic) rather than duplicated structure (loops, conditionals). The refactored solution separates validation and normalization responsibilities into focused functions, keeping control flow simple even if some structural duplication remains. Key takeaway: prefer duplication over wrong abstraction, and only abstract when you notice truly duplicated rules.