well, in this case, they are twins, not duplicates .... and remember nikhila had mentioned about their "missing son" almost at the beginning of the drama.... we just never thought that he might be shiv's twin!
and to give more credibilty to the "lost son" they are probably trying to show that twins run in the family! đ
anyways, i think the way the duplicate twin came in in this drama makes more sense than any other drama... for eg. kasauti, kaajal etc.