I looked a little more. The first time was a normal purchase: you got to the buy screen, then a confirm screen, then the 3rd screen has the completed purchase.
The second purchase didn't show the buy screen or the confirm screen, just the 3rd screen, so it most likely was caused by the back button or a reload of the page or something similar (we have checks in to prevent this, but the two purchases were far enough apart they didn't get noticed as the "same" purchase).
In addition, the same password was used both times, again indicating a "back button" style problem.
Sorry for the difficulty, I'll void the second purchase on monday.
j