Double merles

As previously mentioned, breeding double merles is a risky business that can lead to dogs that are born with various health problems. This author sees no reason why a breeder would intentionally produce a double merle. There is nothing to gain from it, and plenty to lose. If you would like to learn more about the condition, a helpful page can be seen here.
Shannon Ledler has been kind enough to supply me with a couple of photos of her dog, Blink.
Blink is an australian shepherd / spaniel cross, and he is a double merle.

In the first photo in particular, you can see that his eyes appear under-sized. As far as his owner can tell, he is completely blind, and only has about 5-10 % hearing in one ear.
Has Blink been a "normal" merle rather than a double merle, he would not have had these health problems.