I've heard it tons of times from people, media, the internet and everywhere, but I guess I was too afraid to ask what does the first and second it indicate? I assume both are not indicating the same thing because it's plain redundant.
Bonus question: I remember my English teacher said never use the same pronoun (like it) twice in a sentence, especially they are supposed to indicate different things. Can I safely assume "It's worth it" is an exception from this?