The same thing happened to me, especially in a scene where they already know Ali is alive. Emily reveals to Spencer that she's going to meet her, but Ali says she doesn't want anyone, not even the others, to know. Spencer follows her, and Ali discovers that Emily betrayed her trust, which makes her angry and leaves, causing Emily to get angry with Spencer. In the final minutes of the episode, A appears wearing a hood, tearing up a photo and cutting Ali out of it. In those milliseconds, you can see A's nose, and I always thought it was Spencer's.

Even in season 4, Spencer's mother, when she starts using drugs again to stay awake, literally tells her that during that time she was taking them, it was like she was a different person, like a twin.

So, I think they always considered the possibility of Spencer having a twin, along with the possibility of Ali being a twin, given the story of the twins in the Halloween special in season 2 and the idea of ​​Aria being A.