Weird thing happening to IPhone?

My Iphone is going insane. I gave my friend my old iphone 3gs and I have a 4 now and so I never had this problem but when I text her it comes back as my number. She gets the message but she can also read all of my messages when I send them. Does anyone know how to fix this please!

Are you both using IMessage?

Maybe the account set for using IMessage on the 3GS and the 4 are the same.

Check here to configure IMessage: http://www.gottabemobile.com/2013/02/04/how-to-set-up-imessage-on-iphone-and-ipad/

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