Why do coupons say "do not double" on them?

[Grocery Coupon Guide]

When people start couponing, they often wonder why many coupons have
“do not double” or “do not triple” written on them. The answer to this
question is pretty simple and straightforward.

http://www.grocerycouponguide.com/articles/do-not-double-coupons/

Safeway Coupon Policy

http://www.grocerycouponguide.com/articles/safeway-coupon-policy-2010/

CVS Coupon Policy

http://www.grocerycouponguide.com/articles/2010-cvs-coupon-policy/

Kmart Double Coupons

http://www.grocerycouponguide.com/articles/kmart-double-coupons/

Are you a coupon blogger? Consider participating in Penny Experiment:

http://www.pennyexperiment.com/a-call-for-couponers/

Have a wonderful day and best of luck in all your coupon shopping.