Last year, Batman: Three Jokers by Geoff Johns and Jason Fabok, saw multiple versions of the famed Clown Prince of Crime existing at the same time. Sean Murphy and Matt Hollingsworth's Batman: White Knight, however, gave us two different incarnations of his most famous sidekick, Harley Quinn, in one story.
The world of White Knight is set outside of the current DC continuity, featuring a progressively more brutal Batman and a less mysterious Joker, who everyone knows is actually a man named Jack Napier. In an attempt to get rid of Batman and actually improve Gotham, he takes pills that restore his sanity and goes on a legal crusade to take down the city's former hero.
After being released from Arkham Asylum, apparently no longer insane, he returns home to find Harley Quinn, who is very similar to the classic version of the character: hyperactive, shrill, and completely in love with the Joker, despite his abuse towards her for years. He proposes to her, and she reacts to this by punching him in the stomach, begging him to stop taking medication and go rob something. Things get even more complicated when Harley is knocked out by... another Harley?
After this Harley takes Jack back to her apartment, she tells him her backstory, which is similar to her classic origin: she was Harleen Quinzel, a psychiatrist at Arkham who was seduced by the Joker and joined him as his partner in crime. She slowly realized, however, that he didn't love her, and he was only obsessed with Batman. She drew the line when she found Joker beating up the second Robin, Jason Todd, and went to Batman to put a stop to him, but by the time they got back to The Jokers' hideout, Jason was gone. Batman got the courts to go easy on Harley, reducing her prison sentence, and she never went back to Joker. Now that he was Jack, she decides to stand by him in his efforts to clean up Gotham.
The other Harley later reveals her own backstory. A bank worker named Marian Drews says she was a self-harmer, and it had only gotten worse since her boyfriend left her. One day at her job she was slicing her wrists when the Joker began robbing it, putting a gun to her head, and ordering her to help. Finally feeling alive, she assisted him, even going along with him when he started calling her Harley. She followed him to his hideout, expecting him to kill her at any moment, but he instead bandaged her wounds. She says she knew it was Stockholm Syndrome, but it was the only true love she ever felt.
Needless to say, the two Harleys went down very different paths. Claiming to be a "stand-in" while Jack has Joker under control, Marian became the Neo-Joker, and attempted to pull off big crimes with a lot of spectacle to draw her lover out, culminating in using a giant freeze gun to cover all of Gotham in ice. The original Harley became a much more heroic character, joining the GCPD and Batman in stopping Neo-Joker, being the one to actually bring her in. It is even revealed that it was Harley who created the pills to restore Jacks's sanity, meaning she was the one who almost cured the Joker.
Since this run, Harley has become a much more heroic character, currently tracking down a murder alongside the GCPD in Batman: White Knight Presents: Harley Quinn by Sean Murphy and Katana Collins. This is also where Neo-Joker showed up as well and while she is still a criminal, it is implied that she is in a much healthier relationship with Poison Ivy. While they've gone their separate ways for now, we will likely see these two Harley Quinn's cross paths once again.