Reloading a form - ideas needed

[Jan M.J. Storms]

I am wondering if anyone has a solution for this:

I have a html-form for making groups of users (members of a site). If,
after having made a group, the form is reloaded (could happen by
accident) the same action is executed again: another group of users is made.

How can I prevent this from happening? Is there a solution or a workaround?

I have to make the behavior of this form as foolproof as possible.

TIA

Jan Storms