I do not know Geoff Miller and have no opinion. He may in fact be Marine Corps double distinguished but the CMP may not have any records of it. He went distinguished like I did, long before today's connected databases. The same happened to me...
I became a US Marine Corps Distinguished Marksman with the service rifle in 1987. By 1988 I had also earned 22 of the 30 points required to become a Distinguished Pistol Shot. Having both would have made me "double distinguished". I wasn't too concerned with that since I was a rifle shooter at heart. Also, my career took a turn in 1989 when I was selected for and promoted to Warrant Officer. I spent the rest of my career serving as the Marine Corps told me to. Shooting matches wasn't in the Corps' plan for one of the few (about 10) Radar Maintenance Officers and later, Aviation Electronics Maintenance Officer (Ground).
Long retired, in 2015 I resumed shooting NRA and CMP competitive pistol. By the 2016 season I was once again "hard as woodpecker lips" and earned the 3rd Gold and another 10 distinguished points in service pistol. I became a Distinguished Pistol Shot at the Civilian Marksmanship Program's National Matches at Camp Perry, or so I thought.
What's the point of this story? The CMP had NO records of my 22 Marine Corps pistol points. When the CMP manager called out names of those to be awarded their cherished distinguished badges at the awards ceremony, no one called my name. To correct that I had to send copies of my from my service record book Page 11 (administrative remarks) and photos of my badges with my name and year earned on their back to the OIC of the Marine Corps Shooting Teams.
The OIC being a good Marine promptly corrected the administrative oversight of years back. Two weeks later at the National Rifle Championships, there was a special awards ceremony just for me. The Marine Corps Rifle Team snapped to attention and rendered me honors as the OIC presented my Distinguished Pistol Shot certificate and badge. It was appropriate because after all, the woodchuck shooting, Pennsylvania farm boy is after all these years, still a rifle shooter at heart.