anti-duplication

Code duplication creates maintenance nightmares:

• Bug fixes must be applied in multiple places
• Features evolve inconsistently across duplicates
• Refactoring becomes exponentially harder
• Codebase grows unnecessarily large
• Onboarding takes longer (more code to learn)

This skill transforms "implement X" requests into a two-phase workflow:

1. Search phase: Find existing similar implementations (endpoints, components, services, models)
2. Reuse or justify: Either reuse/extend existing code OR justify why new implementation is necessary

The result: Codebases stay DRY, maintainable, and consistent.

<quick_start> <trigger_pattern> When user says "create", "implement", "add", or "build" new functionality, IMMEDIATELY search for existing similar code before writing anything new.

Trigger phrases:

• "Create a new API endpoint for..."
• "Implement a component that..."
• "Add a service to handle..."
• "Build a model for..."
• "Write a function that..." </trigger_pattern>

<basic_workflow> Step 1: Extract search keywords from request

• User: "Create an API endpoint to fetch user profile"
• Keywords: "API endpoint", "fetch", "user", "profile", "GET"

Step 2: Search codebase for similar patterns

PRIMARY: Semantic search (mgrep)

• mgrep for: "API endpoints that fetch user data", "profile retrieval handlers"
• Finds similar implementations by meaning, not exact text

SECONDARY: Literal search (Grep/Glob)

• Grep for: "router.get", "app.get", "/api/user", "profile"
• Glob for: "/user.ts", "/profile.ts", "/routes/"

Step 3: Analyze findings

• Found 3 similar endpoints: GET /api/user/:id, GET /api/user/settings, GET /api/admin/users
• All use same auth middleware, validation pattern, error handling

Step 4: Present reuse options

• Option A: Extend existing GET /api/user/:id to include profile data
• Option B: Create new endpoint using same patterns as existing user endpoints
• Option C: Justify why completely new approach is needed

Step 5: Implement chosen option (with existing patterns as template) </basic_workflow>

<immediate_value> Before anti-duplication skill: User: "Create an API endpoint to update user email" Claude: Writes new endpoint from scratch with different auth, validation, error handling Result: 5th variation of similar endpoint, inconsistent patterns

After anti-duplication skill: User: "Create an API endpoint to update user email" Claude: Searches codebase, finds 4 existing user endpoints with consistent patterns Claude: "Found 4 similar user endpoints. They all use: authMiddleware, validateUserInput, standardErrorHandler. I'll create the new endpoint following this established pattern." Result: Consistent implementation, reused middleware, maintainable code </immediate_value> </quick_start>

Extract key information:

• Type: What are they building? (endpoint, component, service, model, function, utility)
• Domain: What business domain? (user, product, order, payment, auth)
• Operation: What action? (create, read, update, delete, fetch, validate, transform)
• Technology: What stack? (React, Express, TypeScript, SQL, GraphQL)

Example parsing:

Request: "Create a React component to display product details"

Parsed:
- Type: React component
- Domain: Product
- Operation: Display/render
- Technology: React, TypeScript (likely)

Create multiple search approaches to maximize coverage:

PRIMARY: Semantic search (mgrep) Use mgrep FIRST for pattern discovery - finds similar code by meaning:

• "components that display product information"
• "services that handle email sending"
• "validation logic for user input"
• "error handling patterns in API routes"