Payment is charged twice. Find the cause in code that passed review

This article examines a critical issue in a payment service: despite passing code reviews and having successful tests, users experienced double charges. The author analyzes two different scenarios that led to duplicate transactions and examines the error logs in detail. The focus is on the concept of idempotency—the ability of a system to perform the same operation multiple times without changing the result. The material offers specific software solutions to prevent duplicate charges at the architecture and code levels. The article is useful for backend developers and engineers working with financial transactions, as it clearly demonstrates why standard tests are often insufficient to ensure the reliability of payment systems and how to properly build protection against race conditions and network failures.