You shouldn’t need to build anything like that unless you are REALLY bummed out by an occasional duplicate post; I never advise anyone to do anything except accept POSTs to their endpoint directly, so no, I wouldn’t say anything is missing from the docs for handling this rare edge case.
It’s true; you could receive duplicates, yes, but I wouldn’t expect that it happens often enough to need you to do anything about it except ignore them yourself as you respond to the submitters? If it does, we’ve only really seen this in bulk from mobile users (who can leave a page open on their mobile device, and some of those browsers seem to repost form contents triggering multiple submissions), so then perhaps it makes sense to have the redirect flow (but you could do it client side using <meta refresh...>).
Another flow you could use is to use the event-triggered function to process your form submissions: Trigger functions on events | Netlify Docs . That would allow you (should you want to) to do an API lookup of past form submissions (Get started with the Netlify API | Netlify Docs) to avoid “doing whatever it is you don’t want to duplicate”, which wouldn’t require changes to your front-end.