This story is a sequel to The Drowned Princess
There are two parallel worlds, pony and human, that follow one another almost perfectly. Yet one major difference has confounded the human versions of the mane 6: Sunset Shimmer, the pony, has no human equivalent. Without much to go on, they've simply led their lives as normal, waiting for the opportunity to find her, if she even exists at all.
Then one day, Rarity finds her working at a cafe.
And Rainbow Dash finds her at an interleague soccer game.
And Twilight, Pinkie, Applejack, and Fluttershy find her as well.
It doesn't make any sense... it must be impossible. There must be only one human version of Sunset Shimmer. The truth about her existence is somewhere in the minds of these six identical copies... and the truth of the false versions will bring revelations all their own.