There are two separate issues at play here: Issue 1. It is often difficult to tell that a sable merle (ie. a brown dog with a dilution factor) is a sable merle: they can look like washed-out brown dogs. Theoretically, if you don't know that you have a sable merle, and you breed your brown dog to a blue dog (which, honestly, few people do), you may - whoops - end up with double merles. Likewise, rarely (rarely) a black dog that is genetically a merle may only have a little bit of diluted color, so you may not realize that it has the merle gene. Again, you could breed it to a blue dog and - whoops - end up with double merles. This is a scenario where knowledge and experience will help you avoid creating animals that will suffer. Think chess. Issue 2. Breeders may choose to breed a merle dog to a merle dog. They may do this to make a double merle stud dog, which will produce all blue when bred to black bitches (but it's not like there's anything wrong with black Shelties). They may do this because they feel that there is no other mate that is a suitable match (but it's not like we have a tiny pool of Shelties, at least in the US). This is a scenario where - once you have decided to breed merle to merle - no amount of knowledge and experience will help you avoid creating animals that will suffer. Think shooting dice. There seems to be a belief that "experienced" and "knowledgeable" breeders will (magically?) avoid producing deaf and blind puppies in their merle-merle litters. But really, that's just a conflation of Issue 1 and Issue 2. If you know the merle status of both parents, you have all the knowledge you need to make the right and ethical choice.