A hammer falls off the top roof and strikes the ground with a certain kinetic energy. If it fell from a roof twice as tall how would the kinetic energy compare?

The kinetic energy with which the hammer strikes the ground

is exactly the potential energy it had at the height from which it fell.

Potential energy is (mass) x (gravity) x (height) ... directly proportional

to height.

Starting from double the height, it starts with double the potential

energy, and it reaches the bottom with double the kinetic energy.

No it would not compare because if you dropped off a higher building the kinetic energy would be higher than it was before.