I always thought the line was awkward. What was Mickey implying? What was repeating itself? Just the murders? I didn't get it.
Here's how I interpret it. Mickey's whole speech was, "I wouldn't do that (untie Derek) if I were you. Do you really trust your boyfriend? Don't you know that history repeats itself?"
So if you take that whole paragraph, he's saying the history of Sidney's boyfriend being the killer. Of course, at that time Mickey wanted to make it look like Derek was his partner.