认认真真 is the AABB reduplication of
认真 (rènzhēn, 'serious / conscientious'), a productive pattern in Chinese for adjectives that intensifies the quality and adds a vivid, emphatic feel. Other examples:
高高兴兴,
干干净净,
漂漂亮亮,
安安静静. Almost always paired with
地 (de) before a verb (
认认真真地做).
