NEVER duplicate logic. Extract and reuse.

// ❌ VIOLATION: Duplicated validation
function validateRegistrationEmail(email: string): boolean {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}

function validateProfileEmail(email: string): boolean {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email); // Same logic!
}

// ✅ CORRECT: Single source of truth
function validateEmail(email: string): boolean {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}

// Reuse everywhere
const isValidRegistration = validateEmail(regEmail);
const isValidProfile = validateEmail(profileEmail);

// ❌ Bug in tax calculation requires changes in 3 files
// cart.ts:      const tax = price * 0.08;
// checkout.ts:  const tax = price * 0.08;
// invoice.ts:   const tax = price * 0.08;

// ✅ Single source of truth
// tax.ts:       export const calculateTax = (price: number) => price * TAX_RATE;

// ❌ VIOLATION: Similar functions with minor differences
function formatUserName(user: User): string {
  return `${user.firstName} ${user.lastName}`;
}

function formatAdminName(admin: Admin): string {
  return `${admin.firstName} ${admin.lastName} (Admin)`;
}

// ✅ CORRECT: Parameterized
function formatName(person: { firstName: string; lastName: string }, suffix?: string): string {
  const name = `${person.firstName} ${person.lastName}`;
  return suffix ? `${name} (${suffix})` : name;
}