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Petrova. See you soon

Dear readers, I am launching an appeal for TVD. Me and my roommate we have to rack your brains trying to understand one thing, but we are not coming to the head, so I wonder if we are slow or if we own this thing has not yet been explained. PETROVA
THE CURSE ... Why? Let me explain what I understand is that Klaus has tried to kill Katherine, many years ago (after the poor thing had already been taken away a daughter, which I end up somewhere in the future), serves as a doppleganger Petrova to break the curse of the day or night. To break this curse also serve as a vampire, a werewolf and of course the mythical moonstone. But what I wonder is: why did these poor doppleganger Petrova (among other things, I think I have also realized that Katherine is not the original Petrova) are persecuted? Why this curse hanging over them? Why do I need just a doppleganger Petrova? It 'something I have not yet said or just that I did not understand?
I would be very grateful if I helped them to deal with them ...