An anon user can sign the petition with an email address that is already in civicrm, and, for that matter, has already signed the petition.

Anon users should be notified if they are trying to sign up with an email address that is already in use and should be given the opportunity to login if that email address that they are attempting to use is associated with a drupal user.

If an email address has already signed the petition, the anon user should be notified that the email address they are using has already been used and cannot be used again for this petition. (if it can be, then the user could change the comment, or name information associated with that email address thus corrupting the civicrm contact record).