re: Dual Ownership?

It's happened to me before but mine was accidental. I had slabbed a bunch of 2005 Clad JFKs and had sold 1 or 2 on ebay. While adding one of them, I typoed the last digit and submitted. Turned out that the number I'd accidentally entered was the same one I sold on ebay...

What I think has happened in your case is that the person may have removed it from his set but has not unregistered it yet...