It happens in Romance all the time -- and the SF people like to ridicule the genre for it, too. "How could you have thought that hidden baby/cowboy/brothers/female fighter pilot plots wouldn't start to get stale after you flooded the market with it for the last year and a half?"

I'd like to say that it's nice that no one genre has cornered the market...but it's not. Nice, that is. It's depressing that no one seems able to stand up and yell, "Wait! Wait! We don't have to be just sheep! We can publish a lot of different kinds of books, and sell to everybody!"