Hanna Twin Theory
This theory might be a bit of a stretch, but nothing is impossible in PLL. There are A LOT of twin hints in the show, and I doubt the producers put all these twin clues just for fan-service to those who read the books. The producers said they weren’t going to do the twin theory for ALISON (which may still be a lie), but that does not mean they can’t do it for someone else.
I know many people are jumping to the conclusion that Ezra is the one with a twin but I don’t buy it, at least not yet.
Remember Hanna is becoming more of a main focus this season, with Caleb gone, Marlene King said Hanna is going to be stepping up to the plate and taking things/investigations into her own hands without waiting for Spencer or anyone. Plus when Alison revealed herself she told Hanna to remember what she told her, not any of the other girls. So clearly, or at least in my opinion Hanna is coming into the light more, making her a more important character, perfect timing for an evil twin reveal :).
As for why Hanna would have a twin without knowing it, maybe she does know? Maybe her parents kept it a secret from her? Her twin was given up for adoption? Hanna was adopted herself, but her twin was still out there? Who knows.