app\code\core\Mage\Checkout\controllers\OnepageController.php

gistfile1.php

<?php
if (!$session->getLastSuccessQuoteId()) {
    $this->_redirect('checkout/cart');
    return;
}

$lastQuoteId = $session->getLastQuoteId();
$lastOrderId = $session->getLastOrderId();
$lastRecurringProfiles = $session->getLastRecurringProfileIds();
if (!$lastQuoteId || (!$lastOrderId && empty($lastRecurringProfiles))) {
    $this->_redirect('checkout/cart');
    return;
}

// We comment out this line so the session isn't cleared, so it dosen't redirect on refresh.
//$session->clear();
$this->loadLayout();
$this->_initLayoutMessages('checkout/session');