I'm not clear on one critical detail: Is this new person doing your actual job or doing a position identical to it? Were you told that the company is creating a second position like the one you now hold? You said this person was brought in to do your position but you do not say you were fired--?
I assume here that you're worried that you'll train this person and then be fired. It does happen. Meanwhile, I agree with others that you cooperate to the max, act professional to your very core, and are sure that the bosses know you are supporting this new person entirely. Then you have more standing to go to your supervisor and say with a smile how great it is to have Sally or Sam on board to share your duties. Then wait and see whether the reply is a squirmy "Uh, right..." or a clear and firm, "Yes, and we have some great new plans for YOU...."
Frankly, having worked in a couple of offices where there were some quietly underhanded hirings and firings, I would advise that while you are giving things your all at work with a big smile and extra effort, you also get your resume in shape pronto and start putting out feelers for another job. You may not need one; you might just have well-meaning supervisors who are dumb about how to handle "onboarding" of a new hire. But if you suspect there's more to it, be prepared to job-hunt and never, ever let them see you're upset if they tell you one day that your job has been eliminated.
Don't just quit because you feel disrespected but do prepare yourself privately to job-hunt. It's sad that you feel you can't go to your supervisor and say frankly, "Hey, I like Sally and am glad to have her here, but her duties and mine are the same and I'm not clear on where both our jobs fit into the bigger picture here. What's up?" I could have done that with some of my past supervisors and it would not have been a confrontation but a conversation. It seems you feel you cannot do that with anyone at your office--?