I think the black termination fluid was to separate her from one consciousness into two. The guy on the phone kept saying that she would be just her on her own and no longer share in the experience. He didn't say what would happen to the other body. You'd think he could have mentioned what might happen though especially when it became obvious that the younger self was preying on the older one.

And if Sue was a clone why no one noticed that she was a younger version of Elizabeth? It doesn't make a lot of sense since he keeps saying it's her own DNA.