That's because I stumbled upon the original question; at the time my question was not shown to be under suspicion or anything.
What's the right thing to do in that case?
It happened recently: I asked a question and the only comment was to clarify what I was asking. Later that day that I noticed that it was a duplicate, so I apologised for it in a comment, providing a link to the question that was already answered comprehensively. (It was about 24 hours later that my question was flagged as duplicate.)