We investigate the low-energy properties of a generalized quantum sine-Gordon model in one dimension with a self-dual symmetry. This model describes a class of quantum phase transitions that stems from the competition of different orders. This SU(N) self-dual sine-Gordon model is shown to be equivalent to an SO(N)(2) conformal field theory perturbed by a current-current interaction, which is related to an integrable fermionic model introduced by Andrei and Destri. In the context of spin-chain problems, we give several realizations of this self-dual sine-Gordon model and discuss the universality class of the transitions.