Okay, so "confirms" might be a stretch (Lord knows we're not getting any definitive answers until next Tuesday's two-hour finale) but the promo does drop a major clue that Spencer's biological mother, Mary Drake (Andrea Parker), gave birth to identical twins. The new promo, which is less than a minute long, promises to reveal A.D. — and then shows Mona (Janel Parrish) whacking Spencer in the head, while wearing A's favorite outfit, the black hoodie.
So, does that mean Mona is secretly A? Of course not — because the promo then shows Mona meeting who we can only assume is the real A.D. in what appears to be some kind of hospital.
So, why is Mona hitting Spencer in the face? Maybe the answer is — because it's not actually Spencer at all. If we're going to believe that the former A has spent all this time trying to win the favor of the Liars — as she says in the penultimate episode "Farewell, My Lovely" — then I think it's safe to say that she's also trying to take down the person who was torturing them. Mona is literally taking down someone who looks just like Spencer, because she suspects that person of being A.D.!
You can watch the moment go down at the 0:14 mark.
With only one episode left, it's time for the show to finally answer all of our burning questions. Spencer having a secret twin might be the bow that ties all of the show's mysteries together. It's about time.