The more I think about it, the more I realize we have very little information about how doppelgängers originate in the TVD universe. We say Tatia was the progenitor, i.e. the "original" physical manifestation of both Katherine and Elena, because the Original siblings refer to her as the "originator". I take issue with this notion for a few reasons:
- The Original family may be old and know a lot, but they don't know everything. Although Klaus calls Tatia the "originator of the Petrova line" in Bringing Out The Dead, this might only be because Tatia was the one they knew personally, and they assume she is the "originator".
- This is related to how everyone assumed the Originals were the oldest vampires, until we found out about Silas. Maybe we've only been led to believe that Tatia was the originator of the Petrova doppelgängers, when really she could be a doppelgänger herself in a more ancient line.
- It is never mentioned how doppelgängers originate, or more specifically, how the Petrova line of doppelgängers was created. Elijah only mentions that Esther used Tatia's blood in the spell to turn them into vampires; it is never described whether Esther had to first create a line of doppelgängers, beginning with Tatia, or Tatia was already a doppelgänger.
- Esther describes the doppelgänger's blood as "a potent binding agent for a witch's spell" in Do Not Go Gentle. To me, this makes the notion of creating a doppelgänger hard to swallow - if doppelgänger blood is needed to bind powerful spells, how do you create a line of doppelgängers in the first place? It seems to me that would have to be a powerful spell in itself.
- That said, I struggle with whether doppelgängers seem more like a natural occurrence or something that must be created by magic.
- On the one hand, they could be nature's way of providing a binding agent for spells, and by extension a way for spells to be undone to maintain the balance of nature.
- On the other hand, the very notion of a doppelgänger seems unnatural to me - the recurrence of a particular, identical physical appearance? Very supernatural, and confirmed by the Gilbert Ring "rules".
- (Katherine does mention in Katerina that the doppelgänger was created as a way to undo the hybrid curse, but because we've learned more about doppelgängers since then - from an actual witch, not to mention the one who performed the immortality spell and placed the hybrid curse - I can't take what Katherine knows about the mythology as indisputable fact. I need to hear it from a witch.)
- Another thing that I'm not sure where to put: the spells in which Esther used Elena's blood (first to bind the Original siblings together, then to turn Alaric into an original vampire) sound similar to what Elijah describes: "...it was Tatia's blood that we consumed in the wine on the night where our mother performed the spell which turned us into vampires." It's a parallel (consuming the doppelgänger's blood in some sort of potion) that, to me, supports the notion that Tatia was herself a doppelgänger.
So the question of the "originator" or "progenitor" remains shrouded in mystery, at least from where I stand. I would love it if more of the doppelgänger mythology were revealed in the future.