From the note I understand you using AWS SES' SMTP to send your emails, but your users report that they are receiving duplicate emails.
Please note Amazon assigns a unique message ID to every email sent. Look in the full headers of the emails you received for an ID like the following:
If it is the same in both emails you got, then there is some sort of glitch between Amazon and the receiving SMTP server that is causing it to be resent. If the ID is different between the 2 emails, then you are sending it twice. Check link below to see methods to send email over SMTP.
I hope this helps.
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