Question

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?

Answer 1

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.

Answer 2

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