Tragically, this also means that, even after 31 years, I've still never 'get good' enough.

Well, there's good news and bad news.

The good news is that Lemmy is now surrounding your spoilers with the expected Details and Summary tags, and moving the HR means PieFed is able to interpret the Markdown for both spoilers.

The bad news:
It turns out KBIN doesn't understand Details/Summary tags (even though a browser on it own does, so that's KBIN's problem).
Neither PieFed, or KBIN, or MS Edge looking at raw HTML can properly deal with a list that starts at '0'.
Lemmy is no longer putting List tags around anything inside the spoilers. (so this post now looks worse on KBIN. Sorry about that KBIN users)

Firstly, sorry for any potential derailment. This is a comment about the Markdown used in your post (I wouldn't normally mention it, but consider it fair game since this is a 'Fediverse' community).
The spec for lemmy's spoiler format is colon-colon-colon-space-spoiler. If you miss out the space, then whilst other Lemmy instances can reconstitute the Markdown to see this post as intended, Lemmy itself doesn't generate the correct HTML when sending it out over ActivityPub. This means that other Fediverse apps that just look at the HTML (e.g. Mastodon, KBIN) can't render it properly.
Screenshot from kbin:

Also, if you add a horizontal rule without a blank line above it, Markdown generally interprets this as meaning that you want the text above it to be a heading. So anything that doesn't have the full force of Lemmy's Markdown processor that is currently trying to re-make the HTML from Markdown now has to deal with the ending triple colons having 'h2' tags around it.
Screenshot from piefed:

(apologies again for being off-topic)

Update: for LW, this behaviour stopped around about Friday 12th April. Not sure what changed, but at least the biggest instance isn't doing it anymore.

I've been coerced into reporting it as bug in Lemmy itself - perhaps you could add your own observations here so I seem like less of a crank. Thanks.

I've since relented, and filed a bug

okay

I'm only running one process, I'd assume the problem isn't happening for Feddit.dk.

Perhaps. The lemmy.ca post has a comment in from the mander.xyz admin who's only running one, and there's a new comment in this thread saying mander.xyx is one of the instances they see the most duplicates from.

Yeah, that's the conclusion I came away with from the lemmy.ca and endlesstalk.org chats. That's it due to multiple docker containers. In the LW Matrix room though, an admin said he saw one container send the same activity out 3 times. Also, LW were presumably running multiple containers with 0.18.5, when it didn't happen, so it maybe that multiple containers is only part of the problem.

When I've mentioned this issue to admins at lemmy.ca and endlesstalk.org (relevant posts here and here), they've suggested it's a misconfiguration. When I said the same to lemmy.world admins (relevant comment here), they also suggested it was misconfig. I mentioned it again recently on the LW channel, and it was only then was Lemmy itself proposed as a problem. It happens on plenty of servers, but not all of them, so I don't know where the fault lies.

A bug report for software I don't run, and so can't reproduce would be closed anyway. I think 'steps to reproduce' is pretty much the first line in a bug report.

If I ran a server that used someone else's software to allow users to download a file, and someone told me that every 2nd byte needed to be discarded, I like to think I'd investigate and contact the software vendors if required. I wouldn't tell the user that it's something they should be doing. I feel like I'm the user in this scenario.

We were typing at the same time, it seems. I've included more info in a comment above, showing that they were POST requests.

Also, the green terminal is outputting part of the body of for each request, to demonstrate. If they weren't POST requests to /inbox, my server wouldn't have even picked up them.

EDIT: by 'server' I mean the back-end one, the one nginx is reverse-proxying to.