preventing multiple clicks ?

On my website ive got a html page in which a visitor can fill in his/her email adress, when submitted a cgi script will send the visitor an email with a newsletter.

Is there anyway to prevent that the visitor fills in multiple email adresses ?
My cgi script validates the email adresses, but the visitor can click the back button change a letter within the email adress and click the submit button again. any ideas will be welcome

You could record the ip address for each submission in a database and disallow any further submissions from that ip address that are made within the next five to fifteen minutes. That should hopefully block multiple submits from the one person while allowing a second person who is agssigned the same ip address at a different time to submit.