  The duality symmetric but not manifestly covariant action proposed by
Schwarz-Sen is canonically quantized in the Coulomb gauge. The resulting theory
turns out to be, nevertheless, relativistically invariant. It is shown,
afterwards, that the Schwarz-Sen model naturally emerges when duality is
implemented as a local symmetry of sourceless electrodynamics. This implies in
the equivalence of these theories at the quantum level.
