Hi @cestis1,
Happy to know that this plugin is useful in your use case.
For your problem, it may come from mail queue processing executed concurrently at the same time by multiple threads. Thus, two (or more) thread take the same e-mails from queue and send them multiple times.
Here are some questions :
1. Does this double sending occurs for every mails ? (Is this reproductible ?)
2. How did you configure SMTP Mailing Queue advanced parameters ?
Are you using wp_cron (“Don’t use wp_cron” unchecked) or custom cron ?
What are your queue limit and wp_cron interval values ?
3. Do you have wp_cron executed for each of your multisite ?
Moreover, if wp_cron is executed for every sub-site, I think that mails should be stored in a specific sub-site directory in wordpress upload directory.
Could you check if your mails (pending) are stored in /www/wp-content/uploads/smtp-mailing-queue directory or in site sub-directory (ex : /www/wp-content/uploads/sites/*/smtp-mailing-queue)
Hope that we will find a solution for you as soon as possible.
Yours faithfully,
Birmania