We consider the minimum time problem with xed end-points on a finite dimensional manifold M in the case when the dynamics is a ne with respect to the control and the control set is a box in R^m. We are interested in giving sufficient conditions for strong local optimality. Our sufficient conditions are given using Hamiltonian methods, but here the presence of a double switch makes the proof of the local invertibility of the maximized Hamiltonian flow much more tricky.