when i have a :post handler to handle a form submission, how can i make it so if the user presses refresh it doesn't re-send the post?
Ah of course. It's been a decade since I worked on backend rendered applications... the things you forget :(
Yeah, I also re-learned it recently after forgetting long time ago :)
A usual way would be that the post request handler issues a redirect to a page that can safely be refreshed. There's even a small Wikipedia article about that: https://en.wikipedia.org/wiki/Post/Redirect/Get