So as it happened in the books, Ali has a twin called Courtney, and Courtney was mentally insane and was sent to the mental hospital, and switched places with Ali so she could have Ali's perfect life. Then Alison wanted revenge. So the stalked Courtney (who was pretending to be her) and was A. All the while, Alison was pretending to be Alison in disguise as Vivan Darkbloom (for example to Hanna [at the hairdresser]) So the real Alison used Vivan Darkbloom as a screen name while Coutney took the name of Alison DiLaurentis. And then Alison killed Courtney at the sleepover, and was A to Emily, Spencer, Aria and Hanna. And somehow, Mona found out about the twins, and wanted revenge on Courtney too. So they were both A. And Alison asked Mona to reveal to the girls that Mona was A. That's why she alisema "I did everything wewe asked me to" to a girl in a red coat; Alison. And if wewe noticed in A's 'lair', there was an selection of pictures of Courtney, and inayofuata to it, was a blonde wig, identitcal to Alison and Courtney's hair. And what I suspect happened was after Courtney was murdered, Alison dyed to her brown and appeared to Emily, Hanna and Spencer in what is believed to be hallucinations, however in reality Alison puts to wig on so she is completely recognisable as Alison (Courtney) to Emily, Spencer and Hanna.