Magnetic Monopoles and Duality Symmetry Breaking in Maxwell's   Electrodynamics
  It is shown how to break the symmetry of a Lagrangian with duality symmetry between electric and magnetic monopoles, so that at low energy, electric monopole interactions continue to be observed but magnetic monopole interactions become very highly suppressed to the point of effectively vanishing. The "zero-charge" problem of source-free electrodynamics is solved by requiring invariance under continuous, local, duality transformations, while local duality symmetry combined with local U(1)EM gauge symmetry leads naturally and surprisingly to an SU(2)D duality gauge group. 
Jay R. Yablon