this is quite a normal reaction from most girls.
maybe she likes you but she has a bf who you dont know about yet and she feels a little guilt that is why she is bringing a friend along.
-or-
she is still uncomfortable being alone with you. so at least she can talk to her friend just in case you turn out to be a bore.
This happened to me before. What I did was neg her I said "It's already 2006 and you still want to bring a chaperone? Get on with the times."
She went with me alone.
Another did insist on bringing her friend along but eventually her friend left us to ourselves.
If she insists on having her friend tag along for the date, go on with the date. Pay more attention to her friend, make her like you. You would win the approval of her friend and eventually hers. It would be a sure-thing that she would go out with you on a second date, this time it would only be you and her.