1. unfortunately I don’t know why the mail is delivered two times. I can’t reproduce the problem either. Have tried everything – click quickly several times, or similar. – is delivered once and correctly.
2. I think this is unlikely. The mail is marked twice in the backend as sent. And also at the same time stamp. In addition, all other mails, even with many images, are delivered correctly. The problem occurs maybe with 1 out of 25.
I use the SMTP plugin “Post SMTP” since the beginning.
I have now installed the “WP Mail Logging” and wait until something happens again.