Prevent form resubmit after Browser Back button click

One way to prevent this is to use the POST/Redirect/GET pattern.

Instead of directly sending a 200 after receiving the POST request, the server sends a 303 (or 302) redirect instead. The client follows it and gets (via GET) a 200 then. Refreshing this page repeats the last GET, not the previous POST.

For implementation questions, see the Stack Overflow tag "post-redirect-get".