In the question: "Is it worth it?" Isn't it redundant to have the subject pronoun "it" twice?
Because the two uses of "it" refer to different things, the two uses are not redundant.
When using pronouns (or any noun that is indefinite, like "it"), the focus needs to be on what that noun is referring to. If there was a repetition of the reference, then you'd have redundancy. In this case, the two uses of the word "it" refer to two different things. And in fact, this sentence is comparing two things: the first is an action or sum of actions, and the second is a result or sum of results.
You can see this when substituting in definite nouns, such as:
Is "studying for my exam over the weekend" (first "it") worth "missing out on the house party down the street" (second "it").