How to apply only the last coupon entered ?

Hi everybody,

I have several coupons but I want to apply only the last one. But now, the coupons are cumulative.

For example : I have -10% on product and -20% on cart but if the customer enters the -20% cart coupon, it has to apply on the orginal price and not on the price-10%.

Where can I put a code test : I mean on which PHP file can I add a code test to verify if a coupon is already active in the cart ?

Thanks in advance.