This would be considered line breeding. Line breeding is allowed by most registries in the U.S. Line breeding is sometimes purposely allowed by breeders as a way to intensify certain traits.
Line breeding entails breeding father to daughter, mother to son, brother to sister or more distantly related dogs such as cousin to cousin.
While the purpose is to intensify desirable qualities, line breeding may also sometimes intensify negative traits which are genetically inherited such as hip dysplasia, heart murmurs, eye defects, etc.
There are certain risks and therefore should not be carried out by the average breeders and left to the experienced ones who health test their dogs and screen for hereditary defects.