It depends on the branch of service. Army Purple Hearts have an Oak Leaf Cluster as the second award. I think the Marines and Navy use a Gold Star. The Coast Guard uses stars as subsequent awards also.
No one liked the fact that I had oak leaves on my Army ribbons and stars on my Coastie ribbons on the same ribbon bar, even after the explanation of wearing the awards as they were presented.
Definition. A "wound" is defined as an injury to any part of the body from an outside force or agent, sustained while in action as described in the eligibility requirements. A physical lesion is not required, provided the concussion or other form of injury received was a result of the action in which engaged.
Limitations. Except in the case of a prisoner of war, the wound for which the award is made must have required treatment by a medical officer at the time of injury. Only one award is authorized for more than one wound or injury received at the same instant from the same missile, force, explosion, or agent. Prisoners of war, if entitled, will be limited to a single Purple Heart covering the entire period of their captivity.
Subsequent awards are denoted by gold and silver stars.
Sea services (USMC is a sea service) use "stars" to indicate multiple awards of the same medal. Army and Air Force use "oak leaf clusters". When a serviceman is awarded the same decoration on multiple occasions by different services (as Delta 21 experienced), the device used to indicate each subsequent award is dictated by the awarding authority. So oak leaf clusters and stars can appear on the same ribbon.
A few other information items which will be of use in wading through Representative Murtha's medal controversy; there is no time limit for submission of recommendation for the "Purple Heart", and the Bronze "V" device for certain awards such as the Bronze Star is correctly termed the "Combat Distinguishing Device".
These decorations are awarded for service to the country while part of its armed forces. They range from "I was there" to "extraordinary heroism". They are a very personal history and should be never be "used". Those who have received these distinctions should be proud.
Those who falsely claim them should be reviled.