Well, I think I've cracked it - after a few weeks of on-and-off work on making my webmention sending not spam everyone (see https://www.jvt.me/posts/2019/10/30/reader-mail-webmention-spam/ ) - it's now not re-sending them if they're successful. This is a good first step, but I'll be improving it to re-send if the post's data has changed since last time it tried (in the future). Note that this isn't quite done yet, expect it to be live tomorrow perhaps.