As you start developing server software, you quickly find the need to do things asynchronously, by passing messages back and forth between various services. A common use case would be using celery to do job processing outside your web server. Another example might be calling webhooks, and doing other integrations. When architecting these things you might reach to a message queue. Different message protocols are either "at least once", or "at most once" delivery, and there is no "exactly once". In this talk we will discuss why "exactly once" is impossible, and some things you can do to mitigate it and get around the limitation.