On Monday, though, he went a step further, calling U.S. President Barack Obama a "son of a b****" - a comment that prompted Washington to call off a bilateral meeting with Duterte at the Association of Southeast Asian Nations (ASEAN) summit in Laos.
The leader's controversial remarks and hardline stance have earned him the nickname of "Asia's Donald Trump" and now, it appears both politicians may share another similarity. The Republican presidential candidate appears to have softened some of his rhetoric and policy positions in hope of appealing to the wider electorate, and Duterte may have to follow suit, albeit for different reasons.
"He's new to the job. At some point, he's likely to conclude that abiding by decorum and diplomatic norms generally would make it easier to get what he needs from the international community and that these kinds of outbursts only intensify the international spotlight," Orchard pointed out.
The Philippines' former United Nations (UN) envoy Lauro Baja mirrored that view.
"I think the president should be advised on the art of diplomacy and the implications of the strategic importance of approaches to the intricacies of international relations," Baja told local news outlet ABS-CBN on Tuesday.
Aside from Obama, Duterte has also slighted Arab culture, Pope Francis, the UN and the body's chief Ban Ki-moon, the Catholic Church, and U.S. ambassador to Manila Philip Goldberg. During his former role as Davao City mayor, he also joked about raping an Australian missionary and burning the Singaporean flag.
But none of his remarks have cost the Philippines any serious damage yet.
"Duterte is not representative of the Philippines at large, though he does echo the positions of a vocal minority," Gregory Poling, a fellow with the Southeast Asia Program at the Center for Strategic and International Studies, said. "He was elected on a domestic populist platform, not because he is in step with the Philippine electorate on foreign policy."