It is also possible that some SMTP servers might process the email twice for an unknown reason. It already happened to me with another PHP script as my Email server was, and still, requires SMTP Auth in order to be validated.
Patches was required specifically. However, before considering code modifications from includes/functions/functions_email.php file, you'd need to clarify if you also receive two emails from your 'Send email' page under your admin section - > tools - > Send email.
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Alternative issue.
From your Email options, under your admin section - > configuration page, you might of also added your own email address into your dropdown's contact list which allows Zen-Cart to send the email twice. Is it the case ?
Please advise.



