From my nginx access log

your.ip.address - - [21/Feb/2024:06:53:19 +0000] "POST /inbox HTTP/1.1" 200 0 "-" "Lemmy/0.19.2; +https://feddit.de"
your.ip.address - - [21/Feb/2024:06:53:21 +0000] "POST /inbox HTTP/1.1" 400 0 "-" "Lemmy/0.19.2; +https://feddit.de"

This was "https://feddit.de/activities/like/19ab3a68-e9ae-4218-8ca2-d8cc885e96f9" being sent twice.

I've reported this problem to other affected servers that I've seen:
the lemmy.ca post suggests the problem was something to do with running multiple containers with the same index number;
the endlesstalk.org post suggests that re-starting the backend containers is a fix
(their answers will likely more sense to you than to me, as I don't run lemmy).

Thanks! (and apologies for this being in English)