Does she use a stunt double?

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She certainly is flexible.

http://www.cosmopolitan.com/entertainment/celebs/q-and-a/a52064/caity-lotz-interview/

This article will answer you question however to quickly sum it up, most of the fighting done is her, while stunts such as jumping through glass and getting thrown across a room is mostly her stunt double.

She's a martial artist and dancer. She's a WEAPON, so yeah she does most of her stunts and it really helps give the show some action credibility but the only times she doesn't do the stunts is when it's sort of really dangerous

YES, all actors do.
The insurance companies wouldn't let them work otherwise.

Check out The Machine. Her moves are pretty awesome and it's very clearly her doing the fighting.

For every lie I unlearn I learn something new - Ani Difranco

Sounds reasonable. She seems really game and physical, but there's probably a limit to what the studios will let her do for safety (and insurance) reasons.