More of this crossover-y weirdness, though I'm not as fond of this piece as I am the others.

This definitely follows Doctors and Doppelgangers and Vampires and Vortex Manipulators.

Enjoy.

Companions and Copies

"They're identical, Doctor," River said.

"I know," the Doctor replied.

"It's not just a resemblance, they're identical."

"I know."

"How is it possible?"

"They're Doppelgangers," he said. "Copies of an original. They've existed for… as long as anyone can remember. Some will tell you that Tatia was the first, but she was not. The Doppelgangers existed long before her blood was used to bind that Hybrid. And they will exist long after Elena."

"You mean there was another?" River asked.

"Several," he nodded. "Elena Gilbert is just the latest in the line of them."

River studied the Doctor for a long moment. "Why are you following them?" she asked finally. "I ran into Katherine twice with you, and you've met Elena… how many of the others have you met?"

"All of them," he said simply.

"All of them?" River repeated.

"Well, all of them I've been able to track down."

"Why?" she asked.

The Doctor ignored her question and continued fidgeting with the TARDIS controls.

"Doctor, what is it about these girls? Doctor?"

"I don't know!" he exclaimed, and the look on his face was complete uncertainty. He really didn't know.

"That's why you follow them," she said, finally understanding. "You really have no idea who, or what, they are, and you hate it."

"Their blood is powerful. But I don't know why. They are hardly more than human, but they are more. I don't know how they exist, or why, and yes, I hate it."

"You poor man," River said, shaking her head. "How did you discover them? Dozens of identical girls scattered through time, someone was bound to notice, but how did you find them?"

The Doctor was quiet for a long time, before he finally began to tell the story.

"Satra was the first I met."

"Satra," River repeated. She knew that name. "She travelled with you."

The Doctor nodded. "For a short time. It was 1137 and she was hardly 17, but she was desperate to get away. She was running from something." They were always running from something. "So we ran together. Until…"

"Until she died."

"Until she died. I met Katerina very soon after. It was over 300 years later, of course, but it was only a few weeks for me. It was quite a shock. I don't usually see dead people hundreds of years after they died. Katerina had no memory of me though. She was a completely different person. So I searched for any hint at their connection. As it turns out, the thing that Satra was running from was her two year old son, Katerina's ancestor. In my search, I found Lucielle, a third Doppelganger, born in 1301. And Arae in 694. They're everywhere, all over time. They seem to be born every 200 years or so, all in the same bloodline, each one descended from the last."

"And you really have no idea why? Or how?"

"Don't you mock me," he said, wagging his finger at her. She threw her arms up in defense.

"I've met Elena, and I've had the incredible misfortune of knowing Katerina. Tell me about Satra?"

"She was… well, I'm sure you can imagine what she looked like. She was sweet, and like to laugh, and loved to run."

River wanted to ask if he loved her, but she already knew the answer. He loved them all.