How common is it to have a second bone graft prior to the dental implant?

Actually it is a common procedure and it happens when the patient didn't get enough height or width on the first bone graft, or when a lot of time passed after the 1st bone graft.

It is very common. We place a bone graft when we take a tooth out hoping we won't need more grafting when it's time for the implant to be placed. If removing the tooth was difficult or if a lot of time passes between the removal of the tooth and the implant placement, it's often necessary to graft again when the implant is placed.