Causes of Copraphagy

That confusing “C” word in the above heading is the official medical terminology for fecal feasting, and it sounds slightly more sterile, so it’s a more than a little preferable to other popular vernacular: e.g. crap snacking, turd tasting, or wolfing down waste. So we’ll refrain from that sort of language from here on out.

Copraphagy in dogs is possibly a evolutionary remnant from their scavenging days as more wolf-like kinds of canines. It can also be a learned behavior. Where would dogs that eat poop pick up such a habit?

From you.

This is by way of misunderstanding your actions, of course. When a dog sees you cleaning up, it might mistake your intentions. Or perhaps you’ve scolded it in the past for committing a dumping on your duvet. Copraphagy might be their (not so) clever way of removing evidence of the crime.  It could also be that they saw another dog doing the deed and is imitating it. Imitation is the sincerest form of flattery, after all. 
Stop Your Dog From Eating Poop

There are a few options for keeping dog breath to a healthier level of stench. You could treat your dog’s food with a chemical to make its stools less palatable. Commercial products such as 4-BID are available for this specific purpose. Or you could cut out the middle man and sprinkle some cayenne pepper on the stool directly. Of course, if you’re going to go to that much trouble, you might as well just pick up the offending fecal matter and throw it away. That, in fact, might be the easiest solution. 

If you’ve got cats in the same household, you’ll want to limit your dog’s access to the litter box. This can be tricky if you have a dog the same size as the cats. Try putting in a baby gate, or something like it. So long as the cat can still get through without the dog being able to bypass it so easily. You could also try "Shaming":
Whatever you do, don’t scold a dog that eats poop. It may associate the unwanted behavior with your attention. It’s like the old saying, that there’s no such thing as bad press. When it comes to a caretaker’s attention, dog’s may be willing to take a tongue lashing in order to be your focus for a few moments.

In the end it will just take some effort, and perhaps a little creativity to break a cacography inclined animal. Until then, invest in breath mints.