Incoming messages are sometimes received in duplicate.
Generally, mail is collected from the ISP using the POP3 protocol. In some cases, if the ISP detects an SMTP server on the connecting computer, the ISP will send the messages via SMTP. Often, the message is also collected by POP3 in the normal way, so that two identical messages are received.
This can be confirmed by enabling SMTP/POP3 logging and examining the SMTPRCV and POP3RCV log files for duplicate messages.
The solution is to restrict the SMTP server interface to the local network. Double click Settings and select the Interfaces tab. For the SMTP server, select the local IP address for the interface.