Now I'm curious. I don't think there's literally two of them, but this chapter changes some dialogue around that could make that possibility intriguing. The first time Mia and MC met, they didn't recognize each other by sight, but by name. Mia had changed a lot so Han not recognizing her makes sense, but Han didn't seem to have changed much? And she called him the "real Han Taesoo". Innocuous at first, but depending on framing, you could use that line to describe someone you've heard of but never met in person.
Mia was also supposedly adopted, so if she had an actual sister that nobody in her hometown knew about, that would also make sense. Or if it's just DID from trauma. Someone correct me if I'm wrong, but have we ever gotten a PoV from Mia thinking about her past history? Did she ever recognize Hayoon directly?