Ducati had argued the device on the rear swingarm was for cooling purposes.
Dovizioso beat Repsol Honda's Spanish world champion Marc Marquez by 0.023 seconds after a wheel-to-wheel duel in the floodlit night race at the Losail circuit on March 10.
The Court of Appeal, in a decision published by the governing International Motorcycle Federation (FIM), rejected a request to declare the device illegal and ban its use in future races.
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"The provisional race results are confirmed and are declared as final," it said.
The four teams -- Repsol Honda, Aprilia, Suzuki and KTM -- have five days in which they can appeal to the Court of Arbitration for Sport (CAS).
The second race of the MotoGP season is at Argentina's Termas de Rio Hondo circuit this weekend.
(Reporting by Alan Baldwin in London, editing by Ken Ferris)
