Can the "double set" teeth be a hereditary condition in toy dog breeds?

I see this commonly in the toy breeds and it's called retained deciduous teeth. Usually one or more of the canine teeth is retained but it sounds like your dogs incisors are retained. It's not dangerous but usually we pull the retained baby teeth when we spay or neuter a dog. Sometimes these retained teeth increase the risk of dental disease developing later in life. It probably is genetic to some extent. Don't stress about it. Hopefully you are planning to get your girl spayed and ask your vet if he or she thinks the baby teeth should be removed at that time.