Debugging request timeouts

I need help debugging the reason to why sometimes for instance I receive duplicate order confirmation e-mail when I place an order on a specific website, therefore I need your help to try and replicate the issue. The issue doesn't only happen when you place the order, therefore upon registration of a new account etc.. I will show examples. We will be doing various tests, in hope to replicate the issue. The issue doesn't happen frequently, therefore only abruptly. My goal is to understand what's the root cause of receiving duplicate e-mail and how I would possibly be able to replicate the issue. Please make sure you read my project thoroughly to understand what I am trying to achieve. You should have an extent amount of knowledge in network, this shouldn't be a hard task.