I want to explore some things about dopplegangers, their origins & their use in magic. Due to some theories going around about the true origins of the Petrova doppleganer phenomenon, & some misperceptions of story, I would like to break some things down first.
Lets begin with the Petrova phenomenon & its origins.
The originator, Tatia was used in the Mikaelsons vampire ritual. Her blood had been in the wine on the night they became vampires. She also was involved in the Hybrid Curse placed on Klaus, resulting in a line of dopplegangers. Elena, the present day doppleganger has been called upon as a walking blood bag stemming from the original Petrova's involvement with the Mikaelsons. And now with the arrival of Silas, revealing Stefan is his doppleganger or "shadow self," people are beginning to question some things:
Why did Esther use Tatia for the vampire ritual?
Why did Esther use her for the hybrid curse?
Why is Elena's blood so important to magical workings?
Is it possible there is an immortal Petrova?
All these questions have led to the assumption that Tatia was not the originator at all; but, instead, a dopplerganger herself. Some theorists believe that their is an immortal Petrova out there somewhere who gave birth to the Petrova dopplerganger line.
I have broken down a few things for you to help you guys understand the Petrova doppleganger origins.
Klaus & Elijah both were in love with Tatia. She would not choose. So, because she put the brothers at odds, Esther took her. They later found out it was her blood they consumed in the wine on the night they became vampires.
Breaking that down to answer our first question:
I assume Human blood is important in the vampire ritual. The show is centered around it. It is based in every story. I assume that Esther also wanted to link her children toegther, so she would only have to do the ritual one time, on only one of her children. To link them, she needed pure blood. Who would be best for both tasks? Tatia. Why? Esther felt she was ruining the brothers bond. I assumed that is why Esther wanted Elena's blood to link her children in Dangerous Liasons. Witches are crafty with their spells. If it works once, it'll work again. You don't fix it if it ain't broken.
Now, the hybrid curse. When Nik triggered his werewolf gene, he had offset the balance. He was too powerful. Nature would not stand for that. So, the witches (I assume of the village, including Esther & Ayana) put the hybrid curse on him. Once again, here goes the crafty part... They needed to sacrifice a pure human, a vampire & a werewolf & sealed the spell with the moonstone. Who better to sacrifice as a human? Tatia! Esther did not like her.
The curse was meant to keep Klaus as the only one of his kind... to keep him from creating his own race, endangering every species. The only way the witches sought to do this was to keep his wolf side dormant. He was still a hybrid. One of the main rules of Nature (a mysterious unknown force of our universe) is what is done magically, must be able to be undone magically... a loophole. This granted that Nature must ensure that all of the ingredients of the Hybrid curse were available to the world. So, to balance out the witches spell, Nature created the Petrova doppleganger gene. A physical genetic copy of Tatia and her blood. I assume that Tatia's child would be the first to carry it. Since the child was pure human, Nature could change the properties of his/her blood magically, since the gene is magical itself.
Now, what I have gotten from the show (even though no one actually said it) is that when a doppleganger appears, she MUST pass on the gene herself. Katherine did. And Klaus wanted Elena to after he broke the curse.
Lets get back to Nature. Since the curse could be broken, Nature saw to balance itself out by making the blood of the doppleganger a magical "fix-it" for the imbalance of the Hybrid species. This way, when fate came upon Klaus to break the curse, he would kill his only key to producing his own race.
Note: Since we know that Nature intervenes from time to time, I believe that it saved Katherine from Klaus, by ensuring her blood was no longer pure. Why? Because she had already had a child to carry on the gene. If Klaus had sacrificed her in the 1400's, he would have had Elena as in the present day to create hybrids. Some may say it was fate, but I believe Nature was trying to keep that from happening.
When Elena did come along, Nature's plan was for Klaus to break his curse by sacrificing her. That would mean, his key to creating hybrids would be gone (without having passed on the gene) & Klaus would still be the only one of his kind. However, just like Nature can intervene, servants of Nature have free will to intervene as well. Bonnie saved Elena. This ruined Nature's plan. So as a back up plan, The Spirits forced Esther to intervene by killing Elena, before Klaus discovered the truth. That failed. Then Esther was brought back with a plan C. Link her children together & get rid of all vampires, & with them, all hybrids. That failed too. Then Esther had a plan D. Create a super vampire. She needed Elena's blood for two reasons: she needed pure human blood for the vampire ritual; she wanted to bind Ric's life to Elenas. However that plan failed too.
Now to answer why Elena's blood is so potent. It is because she is a magical occurance. She is a human, but her reason for existing is strictly magical. She would not be here if it were not for Nature. And Nature is magical itself. Tatia's blood hed NO magical properties before the hybrid curse. She was just a human vessel. Sort of like the Expression Triangle sacrifices. 12 humans were sacrificed. Their blood held nothing magical. They were a part of the sacrifices because their blood was pure, just like Tatias. Like I said, human blood is a basis for the show.
Note: Now, I would like to believe that Nature intervened again in The Departed, by ensuring that Elena would not pass on the doppleganger gene for Klaus to have more hybrids. She had served her purpose. So, Nature sought to make her infertile through vampirism. The only problem now is that Elena has devampired Katherine, whose blood may be capable of giving Klaus hybrids. She may be able to pass on the gene herself by reproducing. Though, I doubt things will ever turn out good for her.
Now, Silas. With his Stefan doppleganger revelation, he has people looking for a rule for dopplegangers... one origin. People figure, since Stefan stems from an immortal, the Petrovas MUST stem from an original immortal Petrova. And they are right in a sense. The Petrova dopplegangers stem from Tatia. Silas' storyline is totally different than TO & Tatia.
You see, dopplegangers exist for one main purpose: balance. That is their base! Their origins only matter to a certainly degree, because they are created by Nature, not by the originator. It is not the how; it is the way. Although Tatia's blood was used in the Mikaelson vampire transition, Petrova blood was not the key to killing them. Though, it did become the key to creating hybrids. So, it is possible that Nature created the 'Silas doppleganger' to balance itself out, but Stefan's blood may hold other properties. His blood could be the key to undoing Silas' immortality.
My main point is that not all dopplegangers originate from the same magical workings, because they are created by Nature. Why? To serve as balance.
Now, does all of this mean that theorists are wrong? No. Their theories could definitely become fact one day. But I've seen that a lot of people are confused about the origins of the Petrova dopplegangers. And because they are confused, they are just jumping into all types of theories. I want people know the story, so that their theories make sense. You can not shape the story of what we know to fit your theories. But you can fit your theories around what we know.