What should you do if the person you love is also in love with someone else?
Move on and stop wasting your time.
It often happens that we can't choose who we fall in love with. If they like someone else there is nothing you can do. Have you asked this person to be with you? If it is the case that you can't be with this person try to move on, there are plenty of people to choose from, don't give this person the power to hurt you.
It is actually possible to love two people at the same time and this is what your statement means. There is someone else in this person's life that he/she loves, but also loves you. Although the person may not be able to help it you have to make the choice of either sharing this person with someone else or letting them know 'you don't share!' I hope you choose the latter.