I'm running my first sale, and two of the first three people who paid double-entered a 33% off coupon, leaving me 2/3 of revenue down from those orders. I think most people when thinking about running a discount would not assume the discount could be applied twice. Since significant money is on the line, I think it better if I have to go back and uncheck a box to let people save more (I'm being a client advocate!) than if I have to check a box to prevent losing money and just hoping that I catch the error soon enough before word gets around (and I am unnecessarily put in a sales posture of taking something away from a client).