It makes me wonder whether you've got certain plugins installed in Chrome that are triggering multiple submissions. Incognito mode often disables plugins unless you've specifically asked it to enable them in Incognito mode. That could explain the difference you're experiencing.

If it's not you and only your customers who are having the problem, then poll them to find out a complete list of all their browser plugins, and then review the list for similarities and trends.

I also wonder if there are certain addons you've put onto your store/site that might be getting triggered in some peculiar way by certain browsers.
It's curious why there aren't hundreds of others reporting identical symptoms ... thus the suspicion that it's something unique to your own site.