Whilst I putter away at the day job and work on the second novel in the Iron Elves series I am also developing a YA/middle school series.  My agent looked at it and wondered if it was at all similar to another series already published.  Whenever anyone says ‘gee, that sounds just like ______’ your first reaction is probably a short flight off a tall bridge or green-tinged anger that someone would dare take your idea.  A few deep breathes later you read the offending book and grow increasingly more relieved when you discover it’s nothing like yours.  You’re cloning labradoodles while that other guy cloned dinosaurs.  Whew, right?  Not quite.  When my agent shops my series, it won’t be enough to simply say this isn’t like that.  I get submissions daily that sound an awful lot like the submissions I got the day before, and the week before, and just like any number of books already published.  Editors are always eye-deep in manuscripts and looking for the quickest way to reject something.  A ms that arrives sounding similar to something else and not taking the time to explain how and why it’s different and oh so much better is likely destined for the shredder.  You, or your agent, will need to be armed with detail and substance to show how your ms is so much more, and not at all similar…except that it will still tap into that same market and make as much or more money for everyone concerned.  Original, but not so original that it can’t be categorized with fifty other equally original books all about say, hot and sexy vampires (maybe yours are different because your vampires are descendant from fruit bats and are devastating Florida’s citrus crops.)  Write your story, make it the best it can be, but also be aware of what else is out there, and be able to show, with a good-natured chuckle, how unique your story is…while still slotting nicely among all those other unique books on exactly the same thing.