Password Change Error

I ran into the following issue:

I created a new role called Administrator who has assigned to him all possible priviledges under access control.

- Administrator changes password of Authenticated User works fine

- Administrator changes password of another user with Administrative rights results in the following message:

The e-mail address email@example.com is already taken.

and the password change is not made.

I created another role, gave the user that role and changed the password (to eliminate the change of this being caused by the fact that it was a non-standard role) but this works fine. It was only changing the "administrator" password that gave this error message.