Adding/Editing users, unique emailaddress

There should be an extension to the user registration system. Email addresses should be unique to prevent multiple sign-ups using the same emailaddress. At the same time, filling out an emailaddress while registering should be mandatory. Of course this functionality could be toggled on/off by an administrator.

The same goes for editing a user. There should be a check on whether the edited user now has an already existing emailaddress or not, to prevent administrative disasters.