THIS POST OBVIOUSLY CONTAINS SPOILERS FOR PRETTY LITTLE LIARS SO DO NOT PROCEED IF YOU DO NOT WANT ANSWERS. IF YOU EMAIL ME MAD ABOUT SPOILERS I WILL HAUNT YOU AFTER I DIE.
Twincer is real. A.D. stands for Alex Drake, the other daughter of Mary Drake. Right after Mary gave birth to Spencer and the Hastings took her away, Mary gave birth to a second child, who she named Alex. The Hastings never knew about Alex; Mary ended up putting her up for adoption. She was taken in by a family in England, but after she started manifesting "some issues," that family gave her to an orphanage and made her change her last name back to Drake.
Twincer theorists will be gratified by confirmation that Alex was indeed the person meeting Wren at the airport a few episodes ago. Wren is actually the entire reason that Alex even knew about Spencer, which explains why he kept coming around even after everyone in Rosewood stopped caring about him. At some point in the past, Wren ran into Alex at a bar in England and mistook her for Spencer. Wren and Alex started dating, and through Wren, Alex discovered her relatives back in Rosewood.
Alex came back to Rosewood to get revenge on Charlotte's killer (remember that Charlotte and Alex are half-sisters; in flashbacks, we see that they became close as adults). Her other targets were the Liars, who had everything she never did — friends, family, and access to decent psychiatric care. Alex is also the person who's been sleeping with Toby recently, so if you thought real Spencer was too good for that kind of behaviour, then congratulations! Your prize is getting to tell all your friends that you told them so.
Alex's end goal was to completely take over Spencer's life, and by the end of "Till Death Do Us Part," it was clear that she already seemed to think she was Spencer. She'd convinced herself that Toby was in love with her, and had made plans to keep the real Spencer locked up in her dungeon forever. Unfortunately for Alex, enlisting Mona to help was was huge mistake, because as we all know, the only thing you can count on Mona to do is the one thing you don't expect.
Alex thought Mona was helping her but she was really just tracking her down; Mona located her at the house Toby built for Yvonne. The remaining Liars and their guys ran there just in time to break up the Great Twincer Ax Fight of 2017. Toby ID'ed the real Spencer by asking her about a poem, and the one functional member of the Rosewood PD escorted her away.
Once again, however, the Rosewood PD has nothing on Mona, because somehow she got hold of Alex and Mary Drake, then took to them France and hid them in the back of her creepy doll shop. "She can't keep us here forever," says Mary, but Alex knows better. "Of course she can," Alex says. "She's Mona." Game recognise game.