Duplication Review Skill

Purpose

This skill provides automated duplication detection commands and manual search patterns. Use this as a reference for WHAT to check and HOW to detect duplicates—not for output formatting or workflow.

Automated Duplication Detection

bash ~/.claude/plugins/marketplaces/claude-configs/review/scripts/review-duplicates.sh

Uses: jsinspect (preferred) or Lizard fallback

Returns:

• Number of duplicate blocks
• File:line locations of each instance
• Similarity percentage
• Lines of duplicated code

Example output:

Match - 2 instances
src/components/UserForm.tsx:45-67
src/components/AdminForm.tsx:23-45

Manual Detection Patterns

When automated tools unavailable or for deeper analysis:

Pattern 1: Configuration Objects

# Find similar object structures

grep -rn "const._=._{$" --include="*.ts" --include="*.tsx" <directory>
grep -rn "export.*{$" --include="_.ts" --include="_.tsx" <directory>

Look for: Similar property names, parallel structures

Pattern 2: Validation Logic

# Find repeated validation patterns

grep -rn "if._length._<._return" --include="_.ts" --include="*.tsx" <directory>
grep -rn "if.*match._test" --include="_.ts" --include="*.tsx" <directory>
grep -rn "throw.*Error" --include="_.ts" --include="_.tsx" <directory>

Look for: Similar conditional checks, repeated error handling

Pattern 3: Data Transformation

# Find similar transformation chains

grep -rn "\.map(" --include="_.ts" --include="_.tsx" <directory>
grep -rn "\.filter(" --include="_.ts" --include="_.tsx" <directory>
grep -rn "\.reduce(" --include="_.ts" --include="_.tsx" <directory>

Look for: Similar method chains, repeated transformations

Pattern 4: File Organization Clues

# Find files with similar names (likely duplicates)

find <directory> -type f -name "_.ts" -o -name "_.tsx" | sort

Look for: Parallel naming (UserForm/AdminForm), similar directory structures

Pattern 5: Function Signatures

# Find similar function declarations

grep -rn "function._{$" --include="_.ts" --include="_.tsx" <directory>
grep -rn "const._=._=>._{$" --include="_.ts" --include="_.tsx" <directory>

Look for: Matching parameter patterns, similar return types

Duplication Type Classification

Type 1: Exact Clones

Characteristics: Character-for-character identical Detection: Automated tools catch these easily Example:

function validateEmail(email: string) {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}

Appears in multiple files without changes.

Type 2: Renamed Clones

Characteristics: Same structure, different identifiers Detection: Look for similar line counts and control flow Example:

function getUserById(id: number) {
  /_ ... _/;
}
function getProductById(id: number) {
  /_ ... _/;
}

Type 3: Near-miss Clones

Characteristics: Similar with minor modifications Detection: Manual comparison after automated flagging Example:

function processOrders() {
  const items = getOrders();
  items.forEach((item) => validate(item));
  items.forEach((item) => transform(item));
  return items;
}

function processUsers() {
  const items = getUsers();
  items.forEach((item) => validate(item));
  items.forEach((item) => transform(item));
  return items;
}

Type 4: Semantic Clones

Characteristics: Different code, same behavior Detection: Requires understanding business logic Example: Two different implementations of same algorithm

Refactoring Patterns

Pattern 1: Extract Function

When: Exact duplicates, 3+ instances Example:

// Before (duplicated)
if (user.age < 18) return false;
if (user.verified !== true) return false;
if (user.active !== true) return false;

// After (extracted)
function isEligible(user) {
  return user.age >= 18 && user.verified && user.active;
}

Pattern 2: Extract Utility

When: Common operations repeated across files Example:

// Before (repeated in many files)
const formatted = date.toISOString().split('T')[0];

// After (utility)
function formatDate(date) {
  return date.toISOString().split('T')[0];
}

Pattern 3: Template Method

When: Similar processing flows with variations Example:

// Before (structural duplicates)
function processA() {
  validate();
  transformA();
  persist();
}
function processB() {
  validate();
  transformB();
  persist();
}

// After (template)
function process(transformer) {
  validate();
  transformer();
  persist();
}

Pattern 4: Parameterize Differences

When: Duplicates with single variation point Example:

// Before (duplicate with variation)
function getActiveUsers() {
  return users.filter((u) => u.status === 'active');
}
function getInactiveUsers() {
  return users.filter((u) => u.status === 'inactive');
}

// After (parameterized)
function getUsersByStatus(status) {
  return users.filter((u) => u.status === status);
}

Severity Mapping

Pattern	Severity	Rationale
Exact duplicates, 5+ instances	critical	High maintenance burden, bug propagation risk
Exact duplicates, 3-4 instances	high	Significant maintenance cost
Structural duplicates, 3+ instances	high	Refactoring opportunity with high value
Exact duplicates, 2 instances	medium	Moderate maintenance burden
Structural duplicates, 2 instances	medium	Consider refactoring if likely to grow
Near-miss clones, 2-3 instances	medium	Evaluate cost/benefit of extraction
Test code duplication	nitpick	Acceptable for test clarity
Configuration duplication	nitpick	May be intentional, evaluate case-by-case

When Duplication is Acceptable

Test Code:

• Test clarity preferred over DRY principle
• Explicit test cases easier to understand
• Fixtures can duplicate without issue

Constants/Configuration:

• Similar configs may be coincidental
• Premature abstraction creates coupling
• May evolve independently

Prototypes/Experiments:

• Early stage code, patterns unclear
• Wait for third instance before abstracting

Different Domains:

• Accidental similarity
• May diverge over time
• Wrong abstraction worse than duplication

Red Flags (High Priority Indicators)

• Same bug appears in multiple locations
• Features require changes in N places
• Developers forget to update all copies
• Code review comments repeated across files
• Merge conflicts in similar code blocks
• Business logic duplicated across domains

Analysis Priority

1. Run automated duplication detection (if tools available)
2. Parse script output for file:line references and instance counts
3. Read flagged files to understand context
4. Classify duplication type (exact, structural, near-miss, semantic)
5. Count instances (more instances = higher priority)
6. Assess refactoring value:
   • Instance count (3+ = high priority)
   • Likelihood of changing together
   • Complexity of extraction
   • Test vs production code
7. Identify refactoring pattern (extract function, utility, template, parameterize)
8. Check for acceptable duplication (tests, config, prototypes)

Integration Notes

• This skill provides detection methods and refactoring patterns only
• Output formatting is handled by the calling agent
• Severity classification should align with agent's schema
• Do NOT include effort estimates or workflow instructions
• Focus on WHAT to detect and HOW to refactor, not report structure