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Lopez Nunez, 33, who won the 120kg category in both 2008 and 2012, won 6-0 to give Cuba their second gold of the Rio Games.
Russia's Sergey Semenov and Azerbaijan's Sabah Shariati won bronze.
Semenov beat Germany's Eduard Popp 3-0, while Shariati defeated Heiki Nabi of Estonia 6-0.
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Mijain Lopez Nunez won his third Olympic gold medal as he beat Turkey's Riza Kayaalp to win the 130kg Greco-Roman wrestling title.