Order numbers repeating

I have ecart 5.03. I built a site for a large organization over a year ago. When a purchase is made, the receipt with a number is generated, stored in db and emailed. In certain occassions it repeats the number and I think the person gets charged 2x. I think it happens when the person is using same computer, thus same session and they submit but then go back and try a different card. That's the first problem. But the major one is that it's happened on few occassions that the same number is overwritten for two completely different purchasers in completely different locations. So how can they be on same session? The file on DB is overwritten by another. IS THERE A FIX FOR THIS GLITCH FROM WEBASSIST? The clients are complaining to me and asking me to fix. But, of course, I didn't really build the code. I used webassist to generate it. Thank you.