In a world full of microservices and distributed systems, building robust and reliable APIs is of paramount concern. One key concept that plays a vital role in achieving reliability is idempotency. In this article, we'll delve into the world of idempotency, exploring what it is, why it's important, and how it can be implemented to solve the persistent problem of duplicate processing in APIs. Along the way, we'll see how to build an idempotent API in Node.js with Redis.