  We show that every Galileon theory admits a dual formulation as a Galileon
theory with new operator coefficients. In n dimensions a free scalar field in
Minkowski spacetime is dual to a (n+1)-th order Galileon theory which exhibits
the Vainshtein mechanism when coupled to sources and superluminal propagation
even on-shell. This demonstrates that superluminal propagation is compatible
with an analytic S-matrix and causality. For point sources, the duality
interchanges the strongly coupled Vainshtein regime with the weakly coupled
asymptotic regime. The duality is made manifest in the context of the
decoupling limit of bigravity, but is independent of this.
