Accepts multiple accounts with the same email and password, but other user names.

Oh yes, it's not very nice

Without password processing (e. g. LoginToboggan with Set password option unchecked) it is possible to make simple modification of inline_registration_submit function.

Something like this (see // added)

function inline_registration_submit($form, &$form_state) {
  $mail = $form_state['values']['mail']; // added
  $user = user_load_by_mail(check_plain($mail)); // added
  ...
  unset($form_state['values']['status']); // if not unset the user will be logined
 
  if (!$user) {// added
    if (module_exists('logintoboggan')) {
      logintoboggan_user_register_submit($form['register']['form'], $form_state);
    }
    else {
      user_register_submit($form['register']['form'], $form_state);
    }
  }// added
  else {// added
    $form_state['user'] = $user; //added
  }// added

  $form_state['values']['name'] = $form_state['user']->name;
  ...
  
}