For the part of the question of how this might be seen by an average user, as a fairly average user myself, I'd simply think "Clive knew that this question had already been asked because he remembered answering a question like it". It would seem perfectly normal.
Just my point of view naturally, but it seems perfectly natural that people will remember most clearly those questions that they themselves have answered, and therefore that they will be the people most likely to flag up dupes of those questions.
In fact, if the people who had invested time and effort in old questions refrained from flagging up dupes, we'd be reliant on a) people with exceptional memories for questions they'd seen but not interacted with, b) on StackExchange's not-so-reliable search, or c) on google which (hopefully) the asker had already tried. All these are less reliable than people thinking, "That sounds like that question I answered a while ago".