We've seen two tasks indicating that multiple blocks were simultaneously created:
We want to investigate whether multiblocks impacts abusefilter, and the interactivity of blocks. Additionally, this may suggest a solution for T351511: How to deal with redundant block
I'm just going to close this in favor of T390528: Multiple blocks by AbuseFilter without Multiblocks enabled. The issue seems to be mostly with AbuseFilter, with T362403: AbuseFilter blocked a user twice at the same time being an earlier example. Note the latter is from April 2024 (a year ago) so it is not about the recent config change enabling multiblocks on plwiki.
I don't think I am qualified to fix the actual issue, but either way we should keep conversation consolidated on existing tasks.
I mentioned T351511 in a meeting as it sounded similar, and while that should be given thought too, it is a distinct task from the other two.