I have only held off on writing (or at least developing) a story idea once because someone posted something similar. It was Kelly with her story Fan Friction, which I can't even find on AC anymore, so now I'm wondering if she took it down because it was never finished. Anyway, I don't remember all the details anymore, but it was a suspense that played on fan fiction. I'd had a similar-sounding idea that was inspired by Signal to Noise way back in like 2001, but I'd never done anything with it - it was always just one of those ideas I'd had on the back burner to write someday. I never had a solid outline for mine, but it was one of the ideas I was considering trying to tackle after I finished BMS. If I'd gone ahead and written it, it probably would have been a lot different from Kelly's, but it's just as well that I didn't.
I remember Hannah started posting Something Beautiful four days after I started working on Curtain Call, and even though she didn't reveal it for awhile, I got that "cancer story" vibe about it right away and was like, "Are you kidding me?!" But since SB and CC are like two polar opposite takes on the same basic premise, it worked out fine. There was room for two cancer stories that summer (the more, the merrier!), and I enjoyed reading SB as much as I enjoyed writing CC.