The force of gravity between two objects is given by:

where G is the gravitational constant, m1 and m2 are the masses of the objects and r is the distance between them.
In this case we know that the objects are identical, this means that the masses are the same:

Then we have that:

Now, if we double the mass of each object and the distance between them we have that:

Therefore the new force is the same if doubled both the mass of each object and the distance between them.