It boils down to the question, why do people engage here in the first place.
Personally, I don't write for posteriority's sake, but to have a conversation with an actual person, albeit asychronously. I love talking shop. I don't want to be limited to the actual shop I work for to do so.
You and your team created a platform that is versatile and can be many different things to many different people.
The screen real estate is limited and even fresh topics go out of sight very fast, at least if they don't generate a lot of interaction. This "growth pain" with duplicated topics is a symptome of that.
One thing, that is not so obvious to me is how the "trending topics" work. When bulletin boards were fashionable, the sorting order often was that the thread with the newest reply came first. I'm under the impression that "trending" does take other things into account as well.
One thing I personally would find very helpful, is if I could use the tag box on the main page to just filter the topics on the main page. I don't want to unfollow tags, just because I'd temporarily would like to have a more narrow view on the topics. Also, I find it unfortunate that the layout of the topic subpage is so different from the main layout, e.g. there is no tag box on the left which would allow me to switch between one tag at a time in one step (instead I have to go back to main and select another tag).
I'm certain that if dev.to indeed manages tp keep up the spirit of, as you put it, "being kind and helpful to the person requesting help", then such technical concerns will turn out be but minor matters for which solutions emerge over time.
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We strive for transparency and don't collect excess data.