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The doctor’s diagnosis was incorrect although he had done all the due diligence.
The doctor’s diagnosis was incorrect although the doctor had done all the due diligence.
-The pronoun in this sentence is “he,” however “he” refers to the doctor, not the doctor’s diagnosis. As the doctor is not the subject of the sentence, we only know he's who the "he" refers to since the diagnosis could not be the “he” that had done the due diligence.