The vast majority of duplicate closures use already answered questions as duplicates. So ideally that question should have answers that apply to the new question as well. In my observation this is usually the case, though the answer might be more generic.

I have also seen bad duplicate closures that weren't actually exact duplicates. But people talk like this is the only kind of duplicate closure that actually happens. I've no idea if the rate of bad duplicates is so much higher than I observed, or if people are missing that their question is actually answered in the duplicate.

I was extremely active on SO and other SE sites in the late 00s and early 10s. In those days it wasn't too bad, many duplicates were actually duplicates. However that really did start shifting. This is anecdata of course, but even questions that linked to possibel "duplicates" and pre-expalined why it wasn't the same question were often just closed as duplicates of the exact question that was linked. This happened to me several times before I got fed up and moved on with my life.

SO was a really rewarding place to ask and answer questions in those early days. It really is a crying shame what they did to the community by empowering the worst of the community to be the bosses.

> So ideally that question should have answers that apply to the new question as well.

The point is who decides. If you ask a question and I flag it as a dupe, I might think the answers on the other question apply to yours, but only you know whether they solved your problem or not.

> I've no idea if the rate of bad duplicates is so much higher than I observed,

Sure, and neither does SO! They didn't even measure it. They only looked at the signal "does somebody with points think these questions are similar", and discarded the signal of whether the new user got any value out of the site, and I think that's what did them in.