I have a theory.
And it is quite a likely theory, yes it is!
Harriet is a robot. A robot I say!
Every single time she messages me she says:

Love comes and goes but friends stay forever!!!!! says:
i am ok and what have you been up today

In those words exactly.
Also, she's just asked me whether or not I have a girlfriend for the second time. It's like deja vu or something. Pre programmed scripts, man.

So, the conclusion is that Harriet is the first AI to live among humans, pretending to be one of them. I guess the programmers got a bit lazy with the intelligence of the AI.