Are you using only the webmail interface and a browser? Or is the account enabled in an email client like Outlook or Thunderbird? A mobile app? How are you accessing your email?
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It only happens when using xfinity connect throught MS Edge/Windows 10 pro. Never a problem over a period of years until now. Sending email using my Samsung phone and tablet does not duplicate. Noticed it's only outgoing mail that duplicates from the pc.