问PHP在DB中插入重复项

EN

<?php
session_start();
$error=''; // Variable To Store Error Message
if (isset($_POST['register'])) {
//if (empty($_POST['email']) || empty($_POST['hash'])) {
//$error = "<br /> <p style='font-family:talo; color:red; margin-top:10px; font-size:16px;'>* Username or Password is invalid</p>";
//}
//else
//{
// Define all labels on the register form
$firstName=$_POST['firstName'];
$lastName=$_POST['lastName'];
$title=$_POST['title'];
$suffix=$_POST['suffixOne'];
$suffixTwo=$_POST['suffixTwo'];
$email=$_POST['email'];
$employer=$_POST['employer'];
$expertise=$_POST['expertise'];
$hash=$_POST['password'];
$confirmHash=$_POST['passwordConfirm'];
$primaryAddress=$_POST['primaryAddress'];
$secondaryAddress=$_POST['secondaryAddress'];
$city=$_POST['city'];
$state=$_POST['state'];
$zip=$_POST['zip'];
$country=$_POST['country'];
$primaryPhone=$_POST['primaryPhone'];
$secondaryPhone=$_POST['secondaryPhone'];

$connection = mysqli_connect("localhost", "username", "password","DB");


$register = "INSERT INTO user (userID, firstName, lastName, title, suffix, suffixTwo, email, employer, expertise, primaryAddress, secondaryAddress, primaryPhone, secondaryPhone, city, postalCode, hash) 
VALUES (DEFAULT, '$firstName', '$lastName', '$title', '$suffix', '$suffixTwo', '$email', '$employer', '$expertise', '$primaryAddress', '$secondaryAddress', '$primaryPhone', '$secondaryPhone', '$city', '$zip', '$hash')";
if (mysqli_query($connection, $register)) {
   header('Location: index.php');
}
mysqli_close($connection);
}
?>