You can't.   You just have to deal with it when they do.

You can avoid re-processing a submission by embedding a unique ID in your
Form each time it is displayed.   When you process the form, you enter
the ID in a database.  Or, if it's already there, you don't repeat the
processing.

You probably want to expire your database entries after a little time:
an hour should be fine in a typical situation.

If you're already using cookies (e.g. a shoppingcart), an alternative is
to use the cookie as a unique identifier.   This means you also have to
handle the situation where a user deliberately "goes round twice" and
submits the same form with different contents.

If your script may take some time to process, you should also consider
running it as a background job, and returning an immediate
acknowledgement to the user (see above if your "immediate" response
gets delayed until processing is complete in any case).

Some parts © 2009 Advameg, Inc.