I faced a non-unique problem, my API kept getting duplicate requests in rapid succession from time to time. One of those "It works on my machine" issues that I only saw in a production environment and only due to some misbehaving clients. I know APIs are supposed to be stateless and idempotent and all those buzzwords... I had all that and more, however the duplicates kept coming in, causing the need for cleanup scripts and subtle bugs downstream in a large enterprise system.