So who was the other girl?

I don't think Alice was dead, no. I think the other one was some sort of digital copy. Something mysterious hijacks these girls accounts and online lives. What that mysterious thing is (beyond metaphor) I don't know, but it was an interesting way of showing how things can get out of ones control in that line of work.

Alice was some sort of computer virus .... who or what was behind that virus is never answered !

I was thinking it was a CGI (S1m0ne-esque) copy automatically designed by an algorithm of some sort, as opposed to an actual, living person. As Tinker said, it probably compiles all of her online information in order to achieve this. Information such as the archived audio and video footage taken from the sex website (which, as stated in the movie, the website owns), her chat records (which appeared to be immense), and maybe even info from her social media accounts and the like (which would explain the copies knowledge of Alice's brother, for instance).

The copy not actually being a real person seems to be supported by the fact that she doesn't recognize her own face when Alice is looking right at her and talking to her, she doesn't appear to notice when people in the sex chat are talking about them looking like identical twins, the constant glitching of the camera during her shows, how the copy is stupidly tricked into giving out it's account password (a victim of a programming loophole), and, of course, when Alice bashes her face into the desk and an obviously computer generated blood splatter appears on the copies face.

But what about the dead girls? Well...

My guess is that whoever designed this algorithm isn't killing girls. If they were, then why didin't they come after Alice? No. Instead, I'm thinking the algorithm is copying all the girls ahead of time, then only going into effect once it becomes aware of one's death (in order to continue generating income from their likeness). The reason Alice's digital doppelganger may have prematurely taken over could be due to Alice's mock suicide, when she pretended to cut her throat at the beginning of the film. The pre-programmed algorithm perhaps saw that, thought she really died, and went about it's normal copying-dead-girls routine.

The only part about this that really throws me off is Tinker's apparent foreknowledge about how this happens. Either he's involved, he's lying, or my whole theory is broken lol.

Tinker explained it. She was a digital copy.