This error usually occurs if you are using the incorrect checkout page URL. The URL that you seem to be using is the one generated upon accessing the plan specific hosted page successfully. 

You would see the session ID embedded in it and the same URL cannot be used multiple times once the checkout is completed. This URL will hence throw the error - "Your order has already been submitted successfully". 

The right URL that you need to embed on your website is the plan specific URL which can be found under Product Catalog > Plans. Click on Copy URL and you can embed the URL on your website:

For example, the correct plan URL looks like this - 

and the hosted page URL with the token (the page on which customer lands in after successful checkout) would look like this -