Pressing the BACK button in your browser will not force it to go back to the server to fetch that page again. It will simply display its cache for this page.

The canonical way of validating user input server-side is to return the same page to the user with all values intact and the fields which did not pass validation appropriately marked.

Most modules with FormValidator in their name do this more or less automatically or at least provide lots of functions to assist you in that respect.