Yes it is allowed.
Candidate should appear for all papers of the group to claim exemption. He can get 1 or more exemption in single attempt.
But if candidate gets one exemption in attempt then in next exam candidate will not appear in that paper, hence in next exam candidate cannot claim one more exemption as he is not appearing in all papers.
So 1 or more exemptions are possible in single sitting but you cannot claim 1 by 1 exemption in successive exams