Correct me if I'm wrong but aren't aasimars resistant to only certain kind of elemental damage (like cold and lightning, for instance)? I seem to also remember tieflings having resistance to fire (naturally), so I was thinking the same would be true for aasimars. In other words, their resistance isn't "5 to all elemental damage".
Either way, I would say YES, they stack. This is similar to when other classes, when gaining a feat or ability that they already have, are given options like selecting another feat or improving (ie., stacking) the ones that they have.