PHP Inserting Duplicates In DB

Question

I'am attempting to build a register page and the current code runs correctly but I'am getting duplicate inserts on the DB side. I've researched and have tried several different solutions but nothing has worked out yet. I'm hoping its something very simple that I'am missing. How can I prevent my current code from inserting twice?

<?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);
}
?>

Answer 1

If the page is refreshed, or someone hits it again using the "back" button, the data will get resent to the server and thus get inserted twice. You need to redirect the user to another page, or the same page, using the POST/REDIRECT/GET pattern to avoid this. Sending a 303 HTTP response will tell the browser to replace that page in its history and avoid re-sending the posted data.

if (mysqli_query($connection, $register)) {
    header('Location: index.php', true, 303);
    exit;
}