Melfith

You could add the lines below in the ns_widget_mailchimp.class.php :

Just before the line 156

$subscribed = $mcapi->listSubscribe($this->get_current_mailing_list_id($_GET['ns_mc_number']), $_GET[$this->id_base . '_email'], $merge_vars);

Put :

$listMemberInfo = $mcapi->listMemberInfo($this->get_current_mailing_list_id($_GET['ns_mc_number']), array($_GET[$this->id_base . '_email']));
if ($mcapi->errorCode){
 $response = json_encode($result);
 exit($response);
}
else{
 if ($listMemberInfo['success']){
  $result['error'] = 'The email address is already in use';
  $response = json_encode($result);
  exit($response);
 }
}

And just before the line 207

$subscribed = $mcapi->listSubscribe($this->get_current_mailing_list_id($_POST['ns_mc_number']), $_POST[$this->id_base . '_email'], $merge_vars);

Put :

$listMemberInfo = $mcapi->listMemberInfo($this->get_current_mailing_list_id($_POST['ns_mc_number']), array($_POST[$this->id_base . '_email']));
if ($mcapi->errorCode)
 return false;
else{
 if ($listMemberInfo['success']){
  $this->subscribe_errors = 'The email address is already in use';
  return false;
 }
}