  We compute the supersymmetric partition function on L(r,1)xS^1, the lens
space index, for 4d gauge theories related by supersymmetric dualities and
involving non simply-connected groups. This computation is sensitive to the
global properties of the underlying gauge group and to discrete theta angle
parameters and thus distinguishes versions of dualities differing by such. We
explicitly discuss N=1 so(N_c) Seiberg dualities and N=4 su(N_c) S-dualities.
