  Elaborating on previous theoretical treatments of the roughening transition,
we provide the exact mapping of a surface model which undergoes both roughening
and preroughening onto a mixture of unit and double charges living on the
square lattice. Depending on the model parameters, the preroughening transition
of the surface can be either continuous or discontinuous. Using the surface
temperature as a control parameter, the dual Coulomb gas accordingly undergoes
two consecutive phase transitions through equilibrium phases whose dielectric
behavior is analysed in terms of Monte Carlo simulation and exact finite-size
calculations. Right at the preroughening point of the surface model, the
charged mixture behaves like an electric insulator (provided preroughening is
second-order), while showing metallic behavior below and above that
temperature, at least up to the roughening temperature where the conventional
metal-insulator transition occurs.
