  We study a model of massive photons with a parity invariant and non-local
mass term. We identify a discrete symmetry of the classical equations of motion
and show that this symmetry can be thought of as an electric-magnetic duality
valid only in the presence of a non-zero photon mass. We explore this duality
and discuss some features of the theory. We define notions of electric and
magnetic charge that can be computed in any spatial domain as a volume integral
of a density. These integrals can be transformed into integrals on the
boundary. For instance, the electric charge is given by the sum of the electric
flux through the boundary of the domain and a line integral of the dual gauge
field along that same boundary. We show that electric and magnetic charge is
quantized $q_mq_e = 2\pi n m_\gamma/e^2$, where $e$ is the coupling constant
and $m_\gamma$ is the mass of the photon. Classical field configurations
associated with external electric and magnetic charges as well as the classical
forces between electric and magnetic charges are computed.
