Since this was the first result I got, I kept her XD
Meet Not-Applejack!
Or as her real name is, Jacklyn Apples.
Jacklyn is a failed clone of Applejack, created a changeling group that is loyal to Queen Chrysalis. They tried to replicate the same thing Chrysalis done by using hair from Applejack and using it on the Tree of Harmony, but they had to improvise as the tree rejected their attempts.
They created the clone in their lair, and created her via changeling magic.
When she was created, she was deemed an imperfect clone, and her creators experimented and abused her, trying to keep on perfecting the cloning technique.
One day she was able to escape, and she ended up encountering Derpy Hooves, who confused her for the real Applejack. After helping her, she took her to Sweet Apple Acres to show her to the real Applejack, with both of them being confused.
Jacklyn currently stays in Sweet Apple Acres, living with Applejack and the rest of the family (Most ponies just think Jacklyn is another Apple cousin), and Applejack is trying to help her adjust to normal life.
Jacklyn is very shy and nervous around others, and has self-esteem issues, thinking that bad things happening are because of her, or getting worked up over minor inconveniences. She’s also not as strong as the rest of the Apple family either, and she hurts herself easily.