At Singapore casino Resorts World Sentosa’s Lunar New Year lucky draw for high rollers earlier this year, the biggest prize was a bottle of Martell Cognac. Across the harbour at rival Marina Bay Sands, the top sweepstakes winner walked away with a new Mercedes-Benz CLA 200 worth much more. It is one of the incidents that has made the casino duopoly in one of the world’s top gambling venues more lopsided in the past few years. That is putting pressure on the Sentosa Island resort, whose gaming...