If you really like a person but they have someone already what do yoou do?

hahaha, that is so freaky, i have the same problem. But if you know that he likes you, then all you could do is be a better friend and hang out with him all the time. Who knows, he might change his mind. Get to know him more and talk to him more. Because, who really cares if he has a girlfriend, you like him and you should do something about it, don't wait for him to take some actions because trust me, you might lose your chance. So, keep being a better friend and don't give up, cuz i ain't. hope i helped! =)