Well it my understanding that Mona had psychotic break that lead her spilt personality which why the shrink knew who A was. I don't think she's real A but it makes sense that she started A, if you think about once Alison went missing she had no one torment which why she started tormenting those girls

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I still believe there more people involved every season finale will lead to one of the identities. So we know 2 of them.