Hi Michael,
Here are screenshots from WP Mail Logging showing the messages being sent twice, and one with the headers. The only other plugin I have that sends notifications is Smart Forms, and it’s never had an issue. These are definitely from Newsletter. It doesn’t happen on every subscription, which is even weirder….
Mail Logging DIsplay – example 1
Mail Logging DIsplay – example 2
Message Headers – can’t put the whole message for privacy reasons.