The guy who wrote the "Ivy is really a plant clone!" story, later revealed in an interview that he never meant for Green!Ivy to have always been a clone.
Basically, initially Green!Ivy was the real Ivy, who had turned green because her body is funny like that. Then Ivy had adventures with Harley. Then Ivy left, and because she ws worried about Harley, made a plant clone of herself to look after Harley. While she was away, the real Ivy turned pink again, because her body is funny like that. Then clone!Ivy died.
So, the Green!Ivy is this story could very well be the real Ivy :)