Question

if normal force is doubled what happens to frictional force

Answer 1

frictional force is directly proportional to normal reaction .
so, if normal reaction double then frictional force also double . but we know frictional force is self adjusting but limiting . so, frictional force may be double or not depend on how much external force applied on it .

other wise frictional force is doubled when normal reaction is doubled

Answer 2

we know that coefficient of friction is to force
applied
so by this principal friction will be doubled
hope its help u
its my pleasure to give u the answer