She was unintentionally inaccurate.
She was unintendedly inaccurate.
Which one is the better choice/more idiomatic?
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I would definitely say "unintentionally." I've never heard anyone say "unintendedly." If that's a word, it sounds really antiquated.
I'm not sure that I agree with the other answer that says a double negative is bad in this case. Double negatives can make things confusing, but as a native speaker, I don't find your phrase confusing. Depending on the context, I think it could be very appropriate, in fact.