The character that is being touted as "Harley Quinn" is clearly not the one we knew before the reboot.
It's becoming increasingly clear that not only is she a mockery of the original character, she is truly a different entity. Now, the events of 'Batman: Harley Quinn' have never occurred -- the fact that Joker was able to poison her means that she never got powers, meaning she never took Ivy's potion. Speaking of which, she has never mentioned ever meeting Poison Ivy, or working with the Sirens. Actually, she has never given any nod to any history whatsoever besides a (horribly) revamped origin story with Joker.
So, if DC is going to drop the hook that there have been "other Harleys", maybe they should follow through with it and reveal that the corsetted strumpet we are seeing in Suicide Squad is merely some sort of copy or brainwashed clone. Let's see the original Harley Quinn back in action, under a writer that can do her justice.