急匆匆 describes a state of hurry or haste, often with an urgent or breathless feeling. You can use it as an adverb before a verb (急匆匆地跑) or as a predicate after a subject (他急匆匆的). Unlike the plain adverb 匆忙, 急匆匆 emphasizes the visible, restless quality of the hurry — think 'flustered' or 'rushed off one's feet'. The reduplicated form (ABB) adds vividness and is typical of spoken and literary Chinese.