What's the best way to make sure an order can't be made twice when using direct to checkout? I have a form for a client where they can generate an invoice email for the odd manual transaction here and there. In this email is a link that sends the relevant invoice details to their foxycart, using cart=checkout. Since it's an external link there isn't any session type data passed in and it's possible that someone can pay for the same invoice multiple times.
Is there a random value I can pass in or something that will prevent the link from being processed more than once?