>Why would you use a generated primary key and a generated candidate key? If both keys have to be uniquely inforced and both are system generated why would you use two???

Department of Motor Vehicles generates the next available driver's license number [that may be alpha-numeric] which should be unique. This would be my candidate key. I would also generate a surrogate PK that is integer. Same thing with Social Security office. Banking and other account numbers should also be unique, but do they make good PKs? Phone numbers can be candidates, but I would not use them as a PK. Otherwise, I can think of very few reasons to do this.