So I wrote to the similar-story-guy to clear things up, and now it appears that he thinks I'm a liar and a thief.
Egad, Charlie Brown.
He went from accusatory and "shocked" at my taking his idea to passive-aggressive a-hole during the course of the conversation. I feel I could have handled this better, but at least I did delete irritated phrases before sending the messages. Ahem.
As I publish more and teach more writers, I expect this kind of situation will come up more frequently. I imagine that John Scalzi hears from half a dozen writers every day with similar accusations.
Writers: Have you had to deal with such situations? If so, how did you handle it? I'd like to be the paragon of gentlemanly and instructive without telling the accusor to piss off.