Types of Code Duplication (Clone vs Semantic)

TL;DR

Explanation

Clone duplication: identical or near-identical code — always extract. Semantic duplication: two code sections do similar things with different data or context — requires judgement. Rule of Three: tolerate one duplicate, refactor on the third occurrence. Types: Type 1 (exact clone), Type 2 (renamed variables), Type 3 (modified statements), Type 4 (same algorithm, different structure). Tools: phpcpd (PHP Copy/Paste Detector), jscpd, SonarQube. The wrong abstraction is worse than duplication — don't prematurely abstract code that merely looks similar but has diverging requirements.

Common Misconception

Why It Matters

Common Mistakes

• Extracting two pieces of code that look similar but have different reasons to change — they'll diverge and the shared abstraction becomes a problem.
• Not using phpcpd/jscpd in CI to detect growing duplication.
• Applying Rule of Three too rigidly — sometimes the second occurrence warrants extraction if the pattern is clearly stable.

Code Examples

// Same validation logic in 3 controllers — copy-paste:
if (strlen($name) < 2 || strlen($name) > 50) {
    throw new ValidationException('Invalid name');
}

// Extracted to validator on third occurrence:
class NameValidator {
    public function validate(string $name): void {
        if (strlen($name) < 2 || strlen($name) > 50) {
            throw new ValidationException('Invalid name');
        }
    }
}

References

• ↗ https://phpmd.org/rules/design.html
• ↗ https://github.com/sebastianbergmann/phpcpd

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