Why do we believe misinformation more easily when it’s repeated many times?

Summary
What it is
The illusory truth effect describes how, when we are repeatedly exposed to misinformation, we are more likely to believe that it’s true.

Why it happens
The main reason for the illusory truth effect is processing fluency: when something is easy to process (as familiar information is), we tend to assume that means that it is accurate.

How to avoid it
Critical thinking and fact-checking are the best lines of defense against the illusory truth effect.

from https://thedecisionlab.com/biases/illusory-truth-effect