bodenfels on "[Plugin: Contact Form 7] form sent twice using on_submit or on_sent_ok"

When I use on_submit or on_sent_ok in the Additional Settings field to execute some other javascript, my form is sent twice - e.g. I receive two emails.

I don't understand, why this is happening - who can help me?
Thanks!

http://wordpress.org/extend/plugins/contact-form-7/