(CNN)Two Russian curlers have given back their Winter Olympic bronze medals after one of them, Aleksandr Krushelnitckii, tested positive for the banned substance meldonium. Krushelnitckii had filed a case with the Court of Arbitration for Sport (CAS) after an initial positive doping test, but dropped it Thursday after failing a second test. He said while he had not knowingly violated doping rules, it was pointless to contest the results.