Double a number's continued fraction
Wolfram Language (Mathematica), 44 bytes

ContinuedFraction[2FromContinuedFraction@#]&amp;


Try it online!

Mathematica has a builtin! Yay! Mathematica's builtin is super long. Aww.

Mathematica's continued fractions look like {integer, ...prefix, {...repeating}}

-1 thanks to JungHwan Min