Actually happens quite often, at least a dozen times to myself, and I have observed it happen to others as well.

When it happened to me, I was always upgraded to the next class of service. Once the cabin manager collected both boarding passes, then came back and led the other gentleman (roughly 15 years older than myself) up to first class. I was surprised about that, and when the cabin manager came back I commented: "what a coincidence, two HONs in the same seat." He mumbled something in return and became more and more nervous. Ten minutes later he came back, the other gentleman behind him, and said something about "a mixup of identities", guiding me to my seat upstairs. The other gentleman looked very perplexed.