Two's a Crowd (Two Of A Kind, Book 7) by Mary-Kate Olsen
Meet Mary-Kate and Ashley, the Tween Queen Twins! Your new best friends have just arrived...
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!
By Judy Katschke.
From the teleplay by Chip Keyes, David Hoge and Dan Cross.
From the series created by Robert Griffard and Howard Adler.