https://blogs.scientificamerican.com/illusion-chasers/i-heard-it-before-so-it-must-be-true/

Joseph Goebbels, minister of propaganda for the Nazi German government of the Third Reich, understood the power of repeating falsehoods. “If you tell a lie big enough and keep repeating it,” he asserted, “people will eventually come to believe it.” This phenomenon, pervasive in contemporary politics, advertising, and social media, is known in cognitive psychology as the “illusory truth effect.”

Though multiple studies have found that repeated statements seem more truthful than novel ones, the illusion was thought to be limited to uncertain statements, or those in which people had no other information available, such as prior knowledge.