Two's A CrowdPaperback - 1999
Mary-Kate thought her twin sister Ashley would be her best friend forever. But now Ashley has a new best friend! Ashley's spending all her time with Nicole, and Mary-Kate feels totally left out. She has to get rid of Nicole -- and she knows just how to do it.
She pretends to be Ashley and tells Nicole their friendship is over. It works. Everything's back to normal -- until Ashley finds out!