注册验证

每次我第一次未填充任何文本框时，它都会注册到 db 中，就像我将所有文本框留空一样，但是当您第二次这样做时(两者中的任何一次(，那将是警报框弹出的时间......我不知道该怎么办。请帮帮我。这是我的代码：

<?php
ini_set('display_errors', 0);
$email= $_POST['email'];
$user= $_POST['user'];
$password= $_POST['password'];
$image =($_FILES['image']['name']);
$submit= $_POST['submit'];
if (empty($email) || empty($password) || empty($user) ) 
{
    echo "<script type='text/javascript'>
    alert('You did not complete all of the required fields');
    window.location='blah2.php';
    </script>";  
}

if(isset($submit))
{   
    $con = mysqli_connect("localhost", "root", "", "top")
        or die('error in connection'.mysqli_connect_error());
        $q = "SELECT username , email FROM registries WHERE username = ? OR email = ?";     
        $stmt = mysqli_prepare ($con, $q);
            mysqli_stmt_bind_param($stmt, 'ss', $user, $email);
            mysqli_stmt_execute($stmt);
            mysqli_stmt_bind_result($stmt, $user , $email);
            mysqli_stmt_store_result($stmt);
            $result = mysqli_stmt_num_rows($stmt);
    if($result > 0) 
    { 
        echo "<script type='text/javascript'>
              alert('Email address or Username is already taken. Please pick another one.');
              window.location='blah2.php';
              </script>";       
    }   
    else
    {
        $q="INSERT INTO registries VALUES (?,?,?,?)";
        $stmt = mysqli_prepare($con, $q);
                move_uploaded_file($_FILES["image"]["tmp_name"],"uploads/" . $_FILES["image"]["name"]);
                mysqli_stmt_bind_param($stmt, 'ssss', $email, $user, $password, $image);
                mysqli_stmt_execute($stmt);
                mysqli_stmt_store_result($stmt);
                mysqli_stmt_close($stmt);
                header('Location: blah.php');

    }   
}