I have this problem too. I don't interact with many people these days, and when I did was fortunate to be shy, so I didn't often actually greet somebody with the wrong name. Mostly it plagues my television viewing. I couldn't tell any of the blondes on Buffy the Vampire Slayer apart. I can't tell the two teenage boys and the FBI agent on Twin Peaks apart unless they speak. Etc. There's a woman named Elizabeth whom I would often see at conventions, though, and I kept greeting her as Terry, mistaking her for Terry Garey. Sadly that wouldn't be a problem these days, but she was rather grumpy about it in time, and who can blame her.

I suspect this is way more common than generally acknowledged.