The server has to detect that the form was submitted several times. The usual approach is to give each form a unique ID (e.g. a UUID) in a hidden field or as part of the submit URL; and to make sure that each and every ID is accepted exactly once.
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)
In reply to Re^2: How to avoid mulitple submission of form in html