Efimova had been disqualified from competing by swimming governing body FINA between October, 2013 and February, 2015 after testing positive for traces of the anabolic steroid DHEA.
That triggered an automatic Games ban based on the IOC's set of criteria issued for Russian competitors following revelations of widespread state-backed doping across many sports in the country.
CAS say that Olympic ban is "unenforceable", saying an athlete cannot be sanctioned twice for a doping offence. FINA will now have to decide on her eligibility.
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(Editing by: Ossian Shine)
