The mystery of Alison's (Sasha Pieterse) disappearance has been at the heart of Pretty Little Liars since day one, and as we head into the second half of Season 4, we're eagerly awaiting the hows and whys behind her fake death. This being PLL, we're certain the full story will be teeming with twists and turns.

One twist we wouldn't be shocked to see? Ali has a secret identical twin. (No, really!) Someone has to be inside Ali's coffin right? It's possible that whoever they buried was actually Ali's twin! Think that sounds crazy? Here are six clues that make us believe such a surprise isn't as unlikely as it sounds.

It happened in the books. (Spoiler alert for this entire paragraph!) We didn't pull this idea out of nowhere. In the books, Ali does indeed have a twin, Courtney. Courtney was mentally disturbed. She was also the one who was murdered — by Ali! — after she tricked her parents (and the Liars) into thinking she was Ali, causing Ali (the real Ali, that is) to be sent to a mental institution in her place. We doubt the story would play out exactly the same way on the show, but it's possible the writers have decided to use the same idea in a different way, just as they did with having Mona (Janel Parrish) be "A".

Radley Sanitorium. Speaking of the books, in the series, Ali's sister was kept at Radley for a period of time. The introduction of Radley into the show makes it seem even more likely that the twin will appear — though the writers could simply have chosen the same name as a nod to the book fans.

The murderous twin story. The Season 2 Halloween episode (Season 2, Episode 13: "The First Secret"), opened with Ali telling a creepy story about a murderous girl who killed her twin and then was sent to a mental institution. Sure, it could just be a campfire tale, but in Rosewood, anything and everything can be a clue, so we're not ready to write it off entirely.

Family portrait? There was another clue in the Halloween episode: On Ali's wall, there seemed to be a framed photo of Jason (Drew Van Acker) with a pic of Ali and her twin on either side … Unless our eyes were playing tricks on us.

There's more to learn about Ali: Sasha Pieterse teased that there's still a lot to learn about Alison. She also said that everything ties together in a "crazy" way. Sure, she could be referring to the big reveal that Ali faked her death, but a secret twin would qualify as "crazy" to us, too!

The girl in the window. In the pilot of the show, Spencer (Troian Bellisario) spotted a blonde in the window of the DiLaurentis house, and for a moment she thought it was Ali. What if it was Ali's twin, instead? That would explain the confusion.

Did we miss anything? Do you think this theory could turn out to be true? Let us know in the comments below!

Catch the Pretty Little Liars Season 4 winter premiere on Tuesday, January 7, 2014 at 8 p.m. ET on ABC Family.

Rebecca Martin is an Associate Editor at Wetpaint Entertainment and our resident Game of ThronesBreaking Bad, and genre TV expert. Follow her on Twitter and Google+!