We analyze the way in which duality constrains the exact beta function and correlation length in single-coupling spin systems. A consistency condition we propose shows very concisely the relation between self-dual points and phase transitions, and implies that the correlation length must be duality invariant. These ideas are then tested on the 2-d Ising model, and used towards finding the exact beta function of the q-state Potts model. Finally, a generic procedure is given for identifying a

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... ity symmetry in other single-coupling models with a continuous phase transition.