oscommerce coupon loophole

im using a coupon module in oscommerce. the problem im having is people are exploiting a loophole where they can apply the coupon then reduce their shopping baskets and then procedd to checkout again and the coupon still gets applied as its ALREADY IN THE SESSION. So its checking the coupon rules only if its not in the session. Once its in the session it doesnt check the rules again thereby creating a loophole. so when they go to the payment page its says COUPON ALREADY APPLIED TO THIS ORDER (its in the session) then they can proceed to confirmation page as it doesnt check again. we need it to check the rules even if coupon has been applied (ie. in session) to make sure all the rules are still satisfied. ill send u the checkout files and the coupon file so u can examine. please dont ask for ftp ill show u the lines of code if u want