How should you handle code duplication?

• A. Leave duplication in place and rely on tests to catch issues.
• B. Manually duplicate logic in each module to keep context clear.
• C. Ignore duplication if unit tests pass.
• D. Refactor to extract common functionality, use reusable components, and enforce DRY (Don't Repeat Yourself) principles.

Answer: (D)

Reducing duplication and applying the DRY principle is essential. When you copy the same logic in multiple places, you create many copies of the same behavior, which makes maintenance costly and error-prone. If a bug is found or a change is needed, you have to update every copy, and it’s easy for one to be missed or diverge over time. By refactoring to extract the shared functionality into a single function, class, or module, you establish a single source of truth. This keeps behavior consistent across the codebase, makes testing more straightforward because you validate the shared logic in one place, and makes future changes safer and easier. Building reusable components and clean interfaces reduces the amount of code to maintain and improves overall readability. In contrast, leaving duplication behind, duplicating logic again in multiple places, or relying solely on tests to cover everything doesn’t address the underlying complexity and can allow inconsistencies to creep in.