Consider this earlier question and this recent one. They both ask the same thing - how to dissolve lines in qgis - although the earlier gives the option of using other software as well. The answers to that earlier question went the other software route. I answered the recent question with the qgis solution, and flagged it as duplicate with the thought that my answer would be merged with the earlier question.
It was then pointed out to me that the earlier answers did not address the new question as they didn't cover qgis, even though the question was a duplicate. To which I replied that was correct and I'd misread the answers, and therefore deleted my flag comment. Actually, I just confused myself when doing so, as my original thought/logic still stands. Perhaps the commenter didn't consider the merge option just like I forgot that was my intent, or I should have added my answer to the earlier question and not the new one before flagging. And I think some of that confusion also stems from the wording of the duplicate flag, that it already has an answer.
I've looked at a few meta discussions either on or closely related to this topic, most specifically this one and its included references (including some info about 'gardening' which I need to read/view more thoroughly), but they all seem to be aimed at questions with the same answers. One of them talks about updating old questions, but that was mostly about software versions and not new information or alternate solutions. To me (at the moment), it would make more sense to have fewer versions of the same question with as many answers as possible rather than a bunch of the same questions with different answers. I understand that the quality of both question and answer is important and much discussed on the subject of duplicates, and perhaps I'll get more clarity on that with the 'gardening' info mentioned above.
It doesn't help that after deleting my flag comment I then found yet another question which truly was a duplicate, but had already used my flag for that question. All I can say is I'm new and I need more coffee. There are likely also further levels of editing consideration that my rep hasn't exposed me to yet.
Edit 1: It seems my original question has different implications depending on your level of rep, since only higher levels can actually perform a merge, much less be aware of that function and how it works. As a result, this question sort of forks - the original still stands for people with that ability.
For those without, I want to pull out and emphasize an embedded question from above: Should I have added my (different) answer to the earlier question rather than the new one and then flagged the new one; or should I answer the new one, flag it, and assume the next level up will keep the merge function in mind? Or in either case, post an additional comment to the default flag stating that intent?
As it stands, duplicate could apply to either a question or an answer, or both. The flag available, at least at my level, makes no distinction but seems to imply (based on the blog post mentioned below) it is intended for the case of both.