Okay so I'm reading the current Joker story titled the Man Who Stopped Laughing in it their are two Jokers both claiming to be real and for the other to be fake. Now both are very hard to tell apart and survive some crazy stuff.

Harley doesn't think the Joker in Gotham is real and this is what bothers me she's considered like an expert on him despite being wrapped around his finger for years. I got to thinking if Joker has had people running around as him for years and have even believed they are really him. Then how does Harley even know if she's met the real Joker?

This also explains the abuse Joker was literally always a different imposter sometimes a guy who'd like her and other times a guy that couldn't have cared less. I know that's a bit convoluted, but I think this would be a good twist on her and Joker as it'd give Harley a major identity crisis. It'd also be massively insulting to her from Joker if she met the real him and he truly did not know or care who she was.