Well, most people didn't and you're low-key gonna be kicking yourself after realising that a quick browse through Troian Bellisario's Twitter page would've given it away ages ago.
The IRL Spencer/Twincer's Twitter bio reads: "Actor. Writer. Poet. Dancer. Rock and Roll Jesus. (sometimes I play this chick Spencer Hastings on this show Pretty Little Liars). But only sometimes."
Um, that is SUCH an A.D. manoeuvre you've pulled there, Troian.
In a recent interview with Elle, the star revealed that she had known the plot twist since season five and had to keep it a secret from errybody else.
"But for a long time, people would ask, ‘Who's A.D.?' And I'd say, ‘It's me!' And everyone would just laugh. It's kind of great when you have a secret that so few people—only me, Marlene, and a handful of others at that point—know. It means you can hide in plain sight," she said.
Well played, Troian. Well played.