Well....the system as such is made in such a way that you can make a bet only once...just like a poll...u can vote only once and not multiple amt of times.
However, there is an option to allow multiple bets (betting on more than one option)....but then the point of betting is lost as most ppl will make profit only....whereas, the main aim is to make the odds of winning nickels or loosing nickels better...hence we have only one option for placing bet and one time bet.
Best thing is to see when the event is closing and place ur bet just before the event closes so as to improve ur chances of winning