Once in a while customers may receive a single email message more than one time. The messages are identical, with matching content and "Sent" dates. This article explains one common reason for receiving duplicate messages.
How Email Servers Deliver Mail
Assume that we're talking about a message from email@example.com to firstname.lastname@example.org.
After both servers negotiate a connection the mail server for sendingserver.com pushes the message to the receiving mail server.
While holding the connection open receivingserver.com scans the message for spam and viruses.
Once the message has been declared safe for delivery receivingserver.com queues it for local delivery to sue's mailbox.
When the message finally reaches sue's mailbox on receivingserver.com a message is sent back to sendingserver.com that informs it of a successful delivery.
This process works to ensure that all email is properly scanned, delivered and accounted for. However in very rare cases an interruption between step 3 and 4 can cause a message to be delivered more than once.
For example if the connection is interrupted after receivingserver.com queues it for local delivery but before a confirmation can be sent back to sendingserver.com the sending server will try to redeliver the message.
Since the original copy was queued for local delivery it will come through, but since the sending server never received confirmation it will try to deliver again resulting in duplicate emails. This situation is more likely to happen under intense load situations or with high latency network connections. The issues leading to the interrupted connection can originate on either the sending server's end or with the receiving server.
The above situation is temporary and normally only a few copies will be delivered, they can be deleted safely.