  Electric magnetic duality symmetry is well understood in vacuum. For light
propagating through a medium this symmetry is typically broken. We investigate
under what conditions duality transformation is preserved in a linear medium
and employ these conditions to generalise the definition of optical helicity to
a general linear medium. We will discuss some unique properties duality
symmetric media possess, provided they exist and reformulate Maxwell's equation
in such a way that they explicitly show the decoupling of opposite helicities.
The feasibility of constructing a duality symmetric medium is discussed.
