UF Match issue: duplicate Drupal user emails

See http://forum.civicrm.org/index.php/topic,6887.0.html

In Civi 2.1.6 / Drupal 6.9, if you edit the primary email address of an Individual that is linked to civicrm_uf_match & Drupal user records, this also updates the email (uf_name field) of the civicrm_uf_match record and the mail field of the Drupal user record. It appears that there is no check for duplicate Drupal user emails, so editing a Civi contact can result in Drupal users with duplicate emails. I believe this is something that Drupal strenuously avoids, e.g. because of the implications for password reminders. I expect this could be avoided by setting dupe checking rules accordingly in Civi. However it seems that by default, Civi allows something that is at the least strongly discouraged in Drupal.