Help? Reply Please!

I dont think you are over reacting if you said specifically in the invitations that you were already going to make a cake. It may be though that this person got the invite but didnt read it thoroughly and didnt see that bit where you said you were making the cake and may have decided to help you out a bit by making one. you could laugh it off when the guests arrive and say that now they can have a choice of which cake they would like and everybody would probably see the funny side of things especially if its a birthday party. You could say something like 'now your twice as old as you used to be, you get two cakes!' that would be a great way to clear any tensions there may be. The person who has also made a cake might feel totally embarrassed that they made one when you already said you were going to as well so thats something to think about. Either way, I hope it all goes well for you!