In this paper,we extend our previous result from Lévy (2016).We prove that transport equations with rough coefficients do possess a uniqueness property,even in the presence of viscosity.Our method relies strongly on duality and bears a strong resemblance to the well-known DiPerna-Lions theory first developed by DiPerna and Lions (1989).This uniqueness result allows us to reprove the celebrated theorem of Serrin (1962) in a novel way.As a byproduct of the techniques,we derive an L1 bound for the vorticity in terms of a critical Lebesgue norm of the velocity field.We also show that the zero solution is unique for the 2D Euler equations on the torus under a mild integrability assumption.