http://stackoverflow.com/questions/11377259/form-is-resubmitted-when-the-back-button-is-pressed
The above link had some good links inside. I’ve attached them below:
http://stackoverflow.com/questions/3923904/preventing-form-resubmission
http://stackoverflow.com/questions/660329/prevent-back-button-from-showing-post-confirmation-alert
The following link contains info on how to do it server side.
http://bjw.co.nz/developer/general/75-how-to-prevent-form-resubmission