Personally I see someone that is RPing as a named NPC the same way that I see someone that is playing some form of Drizzt. That is to say, I see them to be someone that really wants to try but doesn't have the ability to make a character that is entirely theirs. Now, granted, they could be new at RP and want to figure out how to do it before they work on a character, but I have seen people fall into the trap of always RPing as someone else's creation.
As for what I would do if I saw someone that was RPing as a canon character? I would probably do what was said above. Ignore them to the best of my ability, and not let it be anything more than an interesting story if my character has to interact with them.
"You wouldn't believe it, but I ran into someone that claims they are Cid Garlemond. I am pretty sure they are just crazy, but they were pretty convincing."
As for what I would do if I saw someone that was RPing as a canon character? I would probably do what was said above. Ignore them to the best of my ability, and not let it be anything more than an interesting story if my character has to interact with them.
"You wouldn't believe it, but I ran into someone that claims they are Cid Garlemond. I am pretty sure they are just crazy, but they were pretty convincing."