I see your point, and I think it is understandable to feel somewhat betrayed. You have the right to want your wedding to be special for yourself and your friends and guests, and the fact that your friend is essentially copying your plans is inconsiderate. She should have spoken with you about it first.

That having been said, some of the other answers have indicated from a practical standpoint that it really isn't that big of a deal from your guests' and friends' standpoint. They are there to help you celebrate, they are not that interested in the venue, and they won't be there to "compare" which wedding was better, fancier, first, or anything like that. And, the bottom line is, if this other person is truly your friend, then you need to give that a lot of consideration as well. It certainly is not worth losing or even harming a friendship over this issue. As they say, you can't have too many friends. Just make the best of it if you can, heap best wishes on your friend as she starts her new married life together, and then have the best time possible in your own wedding. Although the wedding is certainly important, it is nothing compared to the quality and length of your marriage. And nobody can copy that. Best wishes to you.