I have regular (> 1/week) instances of multiple (10+) copies of messages being sent via the POP3 interface.  Appearently the client receives an authentication error (at least that is what Outlook reports) and leaves the message in the Outbox, even though the message was sent, causing the client to retry.  

The situation seems to depend on the message.  I can have several messages in the outbox & all but one will be sent properly.  The "stuck" message never clears from the outbox.