You should really talk to the both of them. Apart or together, that's your call, but you need to calmly talk to them and tell you how you feel. Communication is important in friendship.
You could set dates or outings where you just take your bestfriend along, and maintain the closeness.
Remember; it's not always a bad thing that your friend is friends with this other person, but if you're worried you really should talk to them.
You also need to remember that as people grow older sometimes they develop different priorities and interests. It's not always easy, but sometimes it's best to let go.
But that's only in the very dire of situations. If you really want to keep these friends, have them know how you feel. Jealousy is something we all feel, as long as you don't let it get out of hand it's not a terribly bad thing. You just need a little more communication.

Isn't it obvious? You have a clone who's trying to steal your identity!

Talk to your best friend. She should understand. Invite her to your house, just her, and just have fun. If you invite her places and you pick her up, then it would be pretty hard for the other girl to get involved.

Best of luck xx

You're already different from her. No two people are exactly alike. You don't have to change who you are just to be different from other this girl. Your best friend obviously likes the person you are, or else she wouldn't be your best friend. If you try to change too much, you might end up drifting from your friend because you'll no longer be the person she became friends with. Just relax and continue being yourself. I'm sure you have other qualities that this other girl doesn't have.

It seems like you're feeling as if your "territory" is being threatened. You have to realize that it's not always going to be just you and your best friend. As you both get older, you'll find new friends and if you want those friendships to be strong, you'll have to include them in things. However, you can set time for you and your best friend to do things just with the two of you. You just have to make that clear with your best friend. I think setting time for just the two of you will help, but it can't be all the time. If you lose your best friend because 'she thinks the other girl is better than you', then she isn't truly your best friend. If she can replace you at the drop of a hat, she's not a real friend. Relax and enjoy the friends that you have.

spending time with one best friend is good. spending time with two best friends is better! more people you like being around! i can understand there are things you liked to do with just one friend, but maybe it's time to open up and let others join too. it's a new year after all.

It's funny but, my best friend, well, she used to be jealous of me!! When we first met, it had been pretty random. A friend of mine had chosen to have lunch with a group of people I didn't know, I think she knew one of the people at the table. I became friendly with one of the girls who I met and we started to hang out..I didn't have classes with her but I met her at lunch. However, the girl who later became my bff, didn't know me and was a little jealous..she didn't show it, but she was jealous cause the girl I was hanging out with was one of her friends..and she felt like I was taking her from her..since we started hanging during lunches and stuff. She never showed it though. Eventually I got to know her, and we became friendly. Now we're really good friends, best friends. I tell her I'd feel the same way if someone I didn't know started hanging with her all the time so that I rarely saw her. I mean, I don't blame her for being jealous. I just showed up at their lunch table one day and was soon hanging out with one of her friends.

But now we're best friends.

I agree with what others have said! Maybe plan things to do with both friends and maybe plan a time were it's just you and your friend? Also, if she's your friend, I bet she cares about you a lot too! People have more than one friend, although I can understand, it's hard when you want to share that connection for yourself. Maybe talk to them, or just plan things for you and your friend to do alone.

Feeling jealous is a natural feeling, so don't feel bad, so long as you don't treat the other girl badly because of it. Just see what you can do about hanging together or just hanging alone with your friend!

Talk to both about it. Maybe you'll find that it'll be cooler with the three of you

you must talk to your best friend...if she is really your true friend she will understand and probably explain why she always includes the other girl too.after you two talk to each other , you should talk to the girl too and explain the situation.maybe she will understand and she wont stay with you two all the time.of course this means that you should accompaign with her too but dont include her in all the stuff that you and your best friend do together........

hope that everything will be fine.....

You can't forbid her to have other friends but make sure that you're still 'the best one'.