Finding out that Harley Quinn is actually a Legacy Character is just loaded with this. The character fans remember is a blonde psychiatrist in Arkham Asylum who tried to analyze the Joker and ended up turning into his Monster Fangirl. Was that character the very first one or were there others before her? Sure, she seemed to have stayed alive for a long time but, for all we know, she was killed off-screen and replaced with a lookalike. How many fan-girls has the Joker had?
On the other hand, if it is the same one, he suggests the name to her as a play on her own name, so it brings to light whether or not he was planning to kill her in their meetings.
However, that's only if the Joker's taken at his word on that point - for all we know, he just murdered a bunch of women, threw Harley in a cell with their corpses, and made that up as a way of "testing" her. There would've been some evidence of this beforehand, and Bats would've used it to try to convince Harley to turn away from her life of crime. It's more likely just a sick joke from the Joker to spite Harley for replacing him with Deadshot.
...That actually makes perfect sense! "I'm dead for 5 minutes and you f*ck some Billy the Kid wannabe? Fine! Maybe I'll make you think you're replicable!"
Considering that the New 52 universe is a 5-year timeline for Batman, it wouldn't seem likely that Joker had so many sidekicks in that short amount of time. And even if he did, wouldn't Harley have heard about at one point or another?
That's implying he actually succeeded prior to Harley being dunked in the chemicals, although given the Joker is a chemical genius to the point he can adjust his Joker Venom to generate frowns - suggesting he can alter the chemical makeup of his poisons to a ridiculous degree - it's possible he was just screwing with her.
