I have created this code in order to register users in my database. What I cannot manage to do, is to prevent adding the same user again and again. Here is my code:
connectDB();
$safe_fullname = mysqli_real_escape_string($mysqli,$_POST['fullname']);
$safe_email = mysqli_real_escape_string($mysqli,$_POST['email']);
$safe_password = mysqli_real_escape_string($mysqli,$_POST['pass']);
$addStatement="Insert into Users (Fullname,Email,Password,Is_Admin) values ('".$safe_fullname."','".$safe_email."','".$safe_password."','N')";
$result = mysqli_query($mysqli,$addStatement) or die(mysqli_error($mysqli));