Where did the coupon come from?

The problem with some online coupons is that someone gets the coupon code (or scans a physical coupon) and posts in on the Internet. Then, the coupon gets used be dozens or even hundreds of people. To prevent this, they'll sometimes uniquely code the coupon such that it can be used only once.

Now, you'd think that they'd be happy to have as many people as possible buy from them with coupons. But there are all sorts of reasons to restrict coupon use, including getting information on markets.

I assume you received a flyer and entered the coupon code only once. If that's the case, then it sounds like they made a mistake and owe you $5.