  Duality is one of the oldest known symmetries of Maxwell equations. In recent
years the significance of duality symmetry has been recognized in superstrings
and high energy physics and there has been a renewed interest on the question
of local duality rotation invariance. In the present paper we re-visit global
duality symmetry in the Maxwell action and delineate the ambiguous role of
gauge invariance and time locality. We have recently demonstrated that local
duality invariance in a Lorentz covariant form can be carried out in the
Maxwell equations. In this paper it is shown that in the four-pseudo vector
Lagrangian theory of Sudbery a local duality generalization can be naturally
and unambiguously implemented and the Euler-Lagrange equations of motion are
consistent with the generalized Maxwell field equations. It is pointed out that
the extension of Noether theorem in full generality for a vector action is an
important open problem in mathematical physics. Physical consequences of this
theory for polarized light and topological insulators are also discussed.
