How can i tell my friend to STOP dressing like me?!?

lol ewww but seriously that's creeepy. I think you could maybe stop hanging out with her but if you like her other than that maybe when she asks you could change the subject. Or if it's over the phone you shout "coming mom" and say I have to go bye. But if that fails when she asks next time just tell her it bothers you. Good luck!