You might want to modify your coding. The way I prevent this problem from ever happening is this:
1. The user completes information on the form. 2. The form is validated by PHP. 3. If there are errors, then the form is reshown with their values populated. 4. Once all errors are corrected, I process the form and then use a Header("Location: ") to redirect the user to the same page (or another page). This has the advantage of allowing a user to click the back button and seeing their form with their values still intact. This prevents a method post page from being in the user's browser history.