This is an extension of the earlier question on your strengths. If the interviewer has already asked you about your strengths, then asking this question means he/she would like an answer that is more specific to your job. A variant of this question could be, "Why should we hire you?" The answer will help the interviewer compare what you bring to the table vis-a-vis the others. 
Your strategy: Prepare for this by referring closely to the job description. List out your other strengths and connect them to the requirements mentioned in the job advert. 
Sample answer: You could say, "I have already mentioned my strengths, but if you were to ask me about something unique which I bring, I would say it's my attention to detail. Though it can be annoying for others, this trait of mine has saved several situations from turning into disasters in the past." 
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