First example of (Duality=Gauge) is the T-duality exhibited by string theory on a circle. R gets send to ~1/R.

At the fixed point of this transformation, the theory exhibits an SU(2) gauge symmetry and the T-duality transformation is a discrete transformation in SU(2). It is a redundancy at that point, hence at all points.

Second example: in N=4 SYM, the duality comes from large diffeomorphisms of a torus.