Question

Two charges Q1and Q2 are separated by a distance D. if Q1is doubled, the force is:

Answer 1

the force is also doubled

Answer 2

The potential energy will be halved.

Remember when you are talking about at least two charges then the potential energy is to be considered of any one of the charges due to the other.

The expression of potential energy reads: V(r) = ±q1q24πε0r12
where q1 and q2 are the magnitude of the charges
r12 is the linear distance between point charges 1 and 2
ε0 is a constant known as permittivity of free space (for electric fields)
+ if repulsive force
− if attractive force

So, if the distance between them is doubled then the potential energy is halved as V∝1r when rest of the variables are kept constant.

hope it helps