How can i prevent users from registering twice?

[KingWicked]

Hey guys so i recently made a registration form for my website, but i have a big problem users are able to register more than once so please if you can help me here's my register sumbit form.

<?php
    global $users, $core, $engine;
 
    $errors = array();
    $messages = array();
 
    if(empty($_POST["registrationBean_username"]))
        $errors["registration_username"] = "<br/>Please enter a username!";
    elseif(strlen($_POST["registrationBean_username"]) > 25 || !ctype_alnum($_POST["registrationBean_username"]))
        $errors["registration_username"] = "<br/>Please enter a valid username!";
    elseif($engine->num_rows("SELECT null FROM users WHERE username = '" . $engine->secure($_POST["registrationBean_username"]) . "' LIMIT 1") != 0)
        $errors["registration_username"] = "That username is already taken!";
 
    if(empty($_POST["registrationBean_email"]))
        $errors["registration_email"] = "<br/>Please enter an email address!";
    elseif(!preg_match("/^[a-z0-9_\.-]+@([a-z0-9]+([\-]+[a-z0-9]+)*\.)+[a-z]{2,7}$/i", $_POST["registrationBean_email"]))
        $errors["registration_email"] = "<br/>Please enter a valid email address!";
    elseif($engine->num_rows("SELECT null FROM users WHERE mail = '" . $engine->secure($_POST["registrationBean_email"]) . "' LIMIT 1") != 0)
        $errors["registration_email"] = "<br/>That email address is taken!";
 
    if(empty($_POST['registrationBean_password']))
        $errors["registration_password"] = "<br/>Please enter a password!";
    elseif(strlen($_POST['registrationBean_password']) < 6)
        $errors["registration_password"] = "<br/>Please enter a password with more than 6 characters!";
 
    if(empty($_POST['registrationBean_password_confirm']))
        $errors["registration_password_confirm"] = "<br/>Please enter your password again!";
    elseif(!($_POST['registrationBean_password'] === $_POST['registrationBean_password_confirm']))
        $errors["registration_password_confirm"] = "<br/>Please enter a password with more than 6 characters!";
 
    if($_POST['registrationBean_termsOfServiceSelection'] != "true")
        $errors["registration_termsofservice"] = "Please accept the terms of service.";
 
    $return = array(
        "registrationErrors" => $errors,
        "registrationMessages" => $messages);
 
    if(count($errors) == 0) {
        $users->addUser($engine->secure($_POST["registrationBean_username"]),$core->hashed($_POST['registrationBean_password']),$_POST["registrationBean_email"],$_CONFIG['hotel']['motto'],$_CONFIG['hotel']['credits'],$_CONFIG['hotel']['pixels'],1, $_CONFIG['hotel']['figure'], "M","seckey");
        $users->turnOn($engine->secure($_POST["registrationBean_username"]));
        $return['registrationCompletionRedirectUrl'] = "{$_CONFIG['hotel']['url']}/me";
    }
 
    header('Content-type: application/json'); 
    echo json_encode($return);
    exit;
?>

[Luuk]

You cannot prevent that, a user always has the possibility to register with another email address, a fake name, etc

[zooboole]

This is a spinet from phpocean.com (http://phpocean.com/tutorials/back-e...-a-web-site/15):

if (!$Model->emailAlreadyUsed($email)) 
                {
                    if ($Model->addUser($email, $password)) 
                    {
                        session::set('confirmation', 'You have successfully signed up!');
                        // If you want to can send a confirmation email here
 
                    }else{
                        session::set('error_general', 'Something went wrong.');
                    }
                }else{
                    session::set('error_email', 'This email is already used.');
                }
 

The only way is to force a user to use only one email for an account.