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If you can just sit the test because you have covered the materials, it is totally worth it. In Jr year it gives you 1 more AP in your pocket for application to colleges that won’t see your IB exit score.
At my school, AP US History and IB History of the Americas year 1 is the same class, so if you are talking about taking both classes you only need to take one. If you are talking about taking both exams, then you should. Many at my school do this.