Why do so many manufacturers coupons say ';do not double';?

My supermarket regularly doubles coupons up to 99 cents,but they won,t do so when the coupon says do not double.Why do so many manufacturers coupons say ';do not double';?
They might have a special arrangement with the coupon manufacturer to do so.





Here, there is a popular clothing store that doubles certain 15% off coupons to 30% off every couple of weekends because it's a franchise store and they got permission to do so.





I think they print that on all coupons so that a person won't be able to take advantage of the doubling of the coupon at any other store that doesn't have permission to do it.