We would need to look further into this for you, however the access details on file appear to be incorrect or missing. Could you please update these details by visiting your client area, selecting the relevant purchase, then clicking "Review/Update Access Information" under the "Stored Access Information" section.
We look forward to further assisting you.
This said however, the checkout process requires the user to log in and confirm the payment. The only way a payment can be taken twice is to actually login and pay twice
Its worth noting there is a hook running in that process too, so it would be worth disabling that while testing