// Validate the e-mail address, and check if it is taken by an existing user.
      if ((bool) db_select('users')->fields('users', array('uid'))->condition('uid', $account->uid, '<>')->condition('mail', db_like($form_state['values']['mail']), 'LIKE')->range(0, 1)->execute()->fetchField()) {
          // Format error message dependent on whether the user is logged in or not.
          if ($GLOBALS['user']->uid) {
            form_set_error('mail', t('The e-mail address %email is already taken.', array('%email' => $form_state['values']['mail'])));
          }
          else {
            form_set_error('mail', t('The e-mail address %email is already registered. <a href="@password">Have you forgotten your password?</a>', array('%email' => $form_state['values']['mail'], '@password' => url('user/password'))));
          }
        }