Special Offers: Buy X Get Y

The app works great, and it has a feature which stops people from trying to redeem more than one Buy X Get Y offer at a time ... but... if someone has a Coupon Code, they can apply the Coupon on top of the Buy X Get Y offer. Based on this, I can't offer a Buy X Get Y promotion when I already have active Coupon Codes, because people can 'double dip'.