One meaning of word “myth” is something that is a
widely held but false belief or idea
Source: Google, “define myth”
By this definition, then, a “false myth” is something that it is not a “true myth”, that is, it denies the definition of a “myth”. As I see it, it can either be that it is not a “myth” because it is not widely believed, or because it is not false. However, I've seen a lot of times the phrase “false myth” used to emphasize the falseness of the myth. Is this a genuine usage?