Shop Double Charging

Hi,

I have an issue with orders going through twice on a site with Perch Shop implemented (Paypal Express).

It only happens on some orders, and when it does happen the Perch admin has two separate invoices, generated about 2-3 minutes apart. From what I know, the issue is that the customer tries to checkout, says it doesn't work, and then tries again and it goes through (Obviously going through the first time.).

Below I'll show the code to the entire shop process, please let me know if you see anything that I'm doing incorrectly:

The 'Checkout' button on the cart page sends them to the login page.

Login page:

If logged in sends them to /checkout.php

<?php include('perch/runtime.php'); ?>
<?php
  if (perch_member_logged_in()) {
    PerchSystem::redirect('/checkout.php');
  } 
?>

Otherwise displays login/register forms

Checkout page:

<?php
if (perch_member_logged_in()) {
  // your 'success' return URL
  $return_url = 'https://specialtymotorcams.com/success.php';
  $cancel_url = 'https://specialtymotorcams.com/cancel.php';

  perch_shop_checkout('paypal-express', [
    'return_url' => $return_url,
    'cancel_url' => $cancel_url,
  ]);
} 
?>

Success.php page ($return_url):

(This used to be my $cancel_url as well, I thought that the if(perch_shop_order_successful()) would be enough to determine if payment was successful. After the first double charge, I changed the cancel_url to cancel.php to try and fix this issue. This did not fix the issue.)

runtime included
...html...

<?php
  perch_shop_complete_payment('paypal-express');

  if (perch_shop_order_successful()) {
        perch_shop_empty_cart();
        perch_content('Success');
    }else{
      perch_content('Not Successful');
    }
?>

...html...

cancel.php page ($cancel_url):

runtime
...html...

<?php
      perch_content('Not Successful');
?>

..html..

Shop.php config file:

<?php
  return [
    'gateways' => [
      'paypal-express' => [
        'enabled'   => true,
        'test_mode' => false,
        'live' => [
          'username'  => '***************',
          'password'  => '***************',
          'signature' => '***************',
        ],
        'test' => [
          'username'  => '***************',
          'password'  => '***************',
          'signature' => '***************',
        ],
      ],
    ],
  ];
?>

Aaand diagnostics:

Perch: 3.0.12, PHP: 5.4.24, MySQL: mysqlnd 5.0.10 - 20111026 - $Id: 2d8f599f49e5e8d2f5ba7112b5c444b3e2e48cdf $, with PDO
Server OS: WINNT, cgi-fcgi
Installed apps: content (3.0.12), assets (3.0.12), categories (3.0.12), perch_shop_orders (1.2.5), perch_shop_products (1.2.5), perch_shop (1.2.5), perch_members (1.6.2)
App runtimes: <?php $apps_list = [ 'perch_members', 'perch_shop' ];
PERCH_LOGINPATH: /perch
PERCH_PATH: C:\inetpub\wwwroot\Motorcam-live\perch
PERCH_CORE: C:\inetpub\wwwroot\Motorcam-live\perch\core
PERCH_RESFILEPATH: C:\inetpub\wwwroot\Motorcam-live\perch\resources
Image manipulation: GD
PHP limits: Max upload 4096M, Max POST 1024M, Memory: 1024M, Total max file upload: 1024M
F1: 3b606135b33e6a102526838f4152a807
Resource folder writeable: Yes
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php
REQUEST_URI: /perch/core/settings/diagnostics/
DOCUMENT_ROOT: C:\inetpub\wwwroot\Motorcam-live
HTTP_HOST: specialtymotorcams.com

Any help is appreciated, as this is a pretty serious issue. Thanks!!