are they using an Exchange server for sending emails?
In the server behaviors panel, double click the Universal Email server behavior, click the To button to enter the advancxed to settings and set the wait time between emails to something greater.
It could also be that the person submitting the form is pressing the submit button multiple times.
does the form take a little while to submit?
see this tutorial for adding a progress bar:
and this one for preventing the form from being submitted multiple times:
I have a progress bar and e Captcha. The Formular is on http://fwv-basel-stadt.ch/anmeldungessen.php.