Why Your API Might Create Duplicate Records: Understanding Idempotency

Introduction

Imagine clicking the "Create Order" button on an application.

The request reaches the server, and the backend successfully creates the order. However, due to a network issue, the response never reaches your browser.

You think the request failed, so you click the button again.

This time, the server receives another request and creates another order.

Now you have two orders instead of one.

This is a common problem in backend development, especially when working with APIs and distributed systems.

The solution to this problem involves something called Idempotency.

1. What Is Idempotency?

In simple terms, idempotency means that performing the same operation multiple times produces the same intended result as performing it once.

For example:

  • Sending the same request once should create one order.

  • Sending the same request five times should still result in only one order.

  • Repeated requests should not create unwanted duplicate records.

This is particularly important for payment systems, order management, booking applications, and other systems where duplicate operations can cause problems.

2. Why Do Duplicate API Requests Happen?

There are several reasons:

  • Network connection failures.

  • Client-side request timeouts.

  • Users clicking buttons multiple times.

  • Automatic retry mechanisms.

  • Temporary server or infrastructure issues.

The interesting part is that a request can succeed on the server even when the client thinks it failed.

This makes duplicate request handling an important responsibility of backend engineering.

3. How Does an Idempotency Key Work?

One common solution is to use an Idempotency Key.

An idempotency key is a unique identifier sent along with an API request.

For example:

POST /orders

Request Header:

Idempotency-Key: order-request-12345

When the backend receives this request, it checks whether the key has already been processed.

  • If the key is new, the backend processes the request and stores the result.

  • If the same key appears again, the backend returns the previously stored result instead of creating another order.

This helps prevent duplicate operations.

4. Implementing Idempotency in a Backend Application

A typical implementation involves:

  1. Receiving an idempotency key from the client.

  2. Checking whether the key already exists in the database.

  3. Processing the request if the key is new.

  4. Saving the operation result along with the key.

  5. Returning the stored result when the same request arrives again.

For production systems, this process should also handle concurrent requests safely using database transactions and unique constraints.

5. Where Is Idempotency Useful?

Idempotency is useful in many real-world applications:

  • Payment Systems: Prevent duplicate payment processing.

  • E-commerce: Avoid creating multiple orders.

  • Booking Systems: Prevent duplicate reservations.

  • Banking Applications: Avoid repeated financial operations.

  • Distributed Systems: Handle repeated messages and API requests safely.

Conclusion

Building an API is not just about making requests work successfully.

It is also about making sure the system behaves correctly when something goes wrong.

Network failures, retries, and duplicate requests are normal challenges in production environments.

As developers, we should design our backend systems with these situations in mind.

Because a reliable API isn't just one that works when everything goes right. It's one that behaves correctly when things go wrong.