SO(2) symmetry of a Maxwell p-form theory

We find a universal SO(2) symmetry of a p-form Maxwell theory for both odd and even p. For odd p it corresponds to the duality rotations but for even p it defines a new set of transformations which is not related to duality rotations. In both cases a symmetry group defines a subgroup of the O(2,1) group of R-linear canonical transformations which has also a natural representation on the level of quantization condition for p-brane dyons.