From what I understand, most entertainment businesses (movies, t.v., and videogames as well as book publishing) are all about this kind of thing. We can deride it as "copycat" or "Me, too" but you might see it as a logical extension of 'genres'.
Think of it this way: If you're trying to publish a fantasy book, you don't send a query letter to an agent who specializes in mysteries, unless your fantasy book has a very strong mystery element to it. Similarly, if you happen to be writing a book with a "look and feel" that is similar to something already on the market, you might get a better reception if you query an agent who has already marketed something like that.
It's a delicate balance, definitely more art than science, but that's part of the *ahem* agony and ecstacy of being a professional artist: it's hard and ugly work that may come to nothing, but it might... just might... take you further than you ever dreamed.
I mean, a certain Ms. Rowling could certainly agree to that sentiment...
Good luck, regardless!
