People often ask us if we recommend getting a service dog if they already have a pet dog. While we have done that before, it's not ideal, and some organizations won't do it at all. A service dog is supposed to be focused on you and be able to help you with your disability. When a family has a pet dog, though, the service dog will naturally want to play with the other dog he/she will learn that there are plenty of other fun things in the world besides you. This scenario is far from ideal, and it's one reason why we insist that handlers never bring their service dog to a dog park. 

We always emphasize that a service dog needs time off to play (and should not be working all the time), but even in play, the handler should be interacting with the dog, such as playing tug or fetch. This way, even playtime is an opportunity for the dog to learn that "all good things come from you" and that the dog is rewarded for focusing on you and your behavior. 

The problem comes, of course, from the fact that the kind of person who needs a service dog is often already a dog lover, and so they already have a pet dog. That pet might not be appropriate for service dog training, but few people want to give away their pet dog, even to a friend or relative. So we often try to work around the fact that a family already has a dog, but it definitely is not ideal. The ideal circumstances for getting a service dog rarely comes up, though. Often a person's circumstances are complicated by multiple disabilities, or living in a residence with no yard, or living with many children. We always try to set up the person for success (and set the dog up for success), and one way to do that is to limit the amount of complicating factors.