Double spending is so very difficult to do, if at all. I wouldn't even be worried about it. I mean when's the last time this has even happened ? I haven't heard of double spending forma very long time.
Umm, pretty recently? Double spending happens every day, and most people don't really look into it. Why bother anyway? Blockchain.info has a list of all double or triple spent transactions, I will see if I can find it. Double spending is definitely possible and even if it happens infrequently, it must suck for the gambling site owner to have to pay out a win that could have easily been double spent if he lost.