Two equal charges Q are placed a certain distance apart. They exert equal-and-opposite forces F on one another. Now one of the charges is doubled in magnitude to 2Q. What happens to the magnitude of the force each charge experiences?
Both forces are now 2F. It doesn't matter which charge you consider, the force is given by:
Coulomb's law: F = kQ(2Q) / r2
Another good way to explain this is to remember Newton's Third Law, which says that the forces objects exert on one another are equal-and-opposite.