If it was an accident, then cancel the one you didn't mean to apply to. If it wasn't an accident then you may want to tread carefully, or have a good answer if they find out why you are applying to two different fields (sometimes there are good reasons for doing so, but usually it implies indecisiveness about your career). As to whether they will find out, I would defer to someone else. I think they would assume that most applicants are applying to only their specialty and not go seek out this information, but they may stumble upon it for various reasons.