Hi, @emyvyn!
Thanks for getting in touch!
Please note, that the WP Mail SMTP plugin doesn’t trigger email sending itself. It just bridges a connection between your WordPress site and your chosen email mailing service.
For example, you have a contact (or order) form on your website -> the user submits it -> sending of the notification email about the submission is triggered on the form (or other) level plugin -> this email goes to the WP Mail SMTP plugin -> WP Mail SMTP plugin routes this email to the mailer -> mailer will send and deliver it further.
You can enable the Debug Email Sending option (please, take a look: https://a.supportally.com/i/e1cJPW) to figure out who initiates sending of these duplicated emails -> reproduce the issue -> check the backtrace of the “original” and “extra” emails (please, take a look: https://a.supportally.com/i/M8FUBs).
You can also share these backtraces with us so we can help to find out the initiator of the duplicate email.
Kind regards,