SYMPTOMS

A local mailbox is receiving duplicate messages.

CAUSE

There are generally three possible causes of this issue, each of which are explained below:

Filters, groups. lists and redirections

A filter, redirection, lists and groups may cause a duplicate message if the mailbox was already receiving the email. If you have a filter which sends a copy of an email to another address it is useful to have an action for it to add a header so you know if was copied by the filter. If the duplicate messages were original sent to a list or group you should review the members and the recipient addresses the email was sent to at the SMTP level. Using the message tracking utility will help with that. 

Receiving message timeouts through proxy

A remote mail server sends a message to the mail server and timeouts occur during the DATA command. This may occur if you have an antivirus proxy or similar, where it checks the email before sending the acknowledgement to the sender. What happens is if the proxy is taking too long to check the message, the sender is timing out and dropping the connection, to retry the message at a later time. But the proxy still forwards the email through to the mail server. This problem is usually identified since an email is continually duplicated, not just the once, a new copy keeps arriving, with different Received header, and the problem emails are larger in size than usual.

Third Party Pickup Event or Mailbox Delivery Event

If third party MTA pickup events or mailbox delivery events are being used, message duplications may occur if the pickup event is failing or has not correctly handled its queue management. Try disabling third party plugins and delivery events if these are configured.