Thank you for suggesting these settings changes.
I see jobs collection in Sidekiq and I’ve been killing them regularly. This seems to silence the duplicate emails for a while. But then the problem will reoccur.
I suppose this could be an issue between my Mail server and Discourse/Sidekiq but I’m not sure how to begin diagnosing that. As I say, the problem happened when I updated Discourse. I made no other changes so I’m leaning towards the idea that some change in Discourse has triggered this.