Can you prevent the same person submitting a Webform multiple times?

I have a webform for anonymous users to submit.

However, I don't want multiple submissions from the same e-mail.

Essentially I want the form to throw up a message like "You have already submitted this form!" if a user tries to submit the form any more than once.

(This is automatically handled if the user is registerd, but I want it for anonymous users.)

Any ideas?

Thanks.