Daily withdrawal has been made for today...
https://blockchain.info/tx/48bb942bbe7506b75983702ab56e5bae6d56ffebc4436fef8616a9679f643518Strange.
Look at the times of your bets - if it would be doubled by webpage / browser, your bets would be separated by seconds. They are however separated by minutes.
Perhaps you press 'F5' to refresh page few minutes after your bet? You should rather click through page (click on the 'Dashboard' link for instance), or logout to make sure nothing would go wrong.
Thanks for pointing out the time difference. I did not notice that. In this case, I think, your guess is right. He refreshed and ignored the form re-submission warning, that is expected to be given by the browser.
Creators could add a random field of type 'hidden' to bet form, and register their usage, not letting bets with already used ID.
New bets are generated with new ID. The random number field that you are suggesting will generate a new random number for each form submission. How the system will know whether the new bets are due to intentional or unintentional form submission ?