Just tell her,
"I was trying to warn my friend about the stalker that's been following me. But somehow my stalker started intercepting my texts too!
"Or
"Sorry-sorry, I had quite the drinking-drinking binge, and I think-think I was seeing double-double."
"What? How did you not know you had an identical twin, with the same name, that only I could see??"
"That was a private message. Have you been going through my phone again? How can I learn to trust you if you keep sneaking around and doing these things? That's it, I'm breaking up with you. Tomorrow I'm coming over and picking up my C.D.'s.. Don't try and change my mind either."
Good luck
JONAS