That's what I thought. Well you're more well-versed in Drupal than I am. Whichever you think is best. To be honest, now that I think about it, I think we've only had a few duplicate orders and I think it's been caused more by credit cards being declined, and someone submitting again (after reloading the checkout page perhaps). But I guess it's better to be safe than sorry, so long as it doesn't break the checkout process altogether

I will test your contrib once it's up.