Shared logic is extracted into reusable modules

Duplicated logic is a maintenance trap: every bug fix, every business rule change, and every refactor must happen in N places simultaneously. Miss one copy and you ship an inconsistency. ISO 25010 maintainability.reusability directly penalizes this pattern. AI-generated codebases accumulate duplicate logic across sessions because the model rewrites familiar patterns rather than importing the utility it generated three sessions ago. Date formatting, currency conversion, and API error handling are the most common duplicated patterns.

Extract duplicated utility logic into a shared module and replace each copy with the import:

// lib/utils/format.ts
export function formatCurrency(amount: number, currency = 'USD'): string {
  return new Intl.NumberFormat('en-US', { style: 'currency', currency }).format(amount)
}

export function formatRelativeDate(date: Date): string {
  // single implementation, used everywhere
}

Your IDE's "Extract to function" refactor handles the initial extraction. After centralizing, run a search for the old inline pattern to confirm all copies are replaced.