Why do I keep getting duplicate emails in Outlook?

This will happen anytime a POP3 session does not complete successfully.  Here is a practical example of how this can happen.
If you had 20 messages in your mailbox on the server and you begin downloading them and for ANY reason the session does not complete (meaning you do not get all 20 message) the next time you attempt to download messages you will get all of the ones already download and any new ones that might have arrived since your last attempt. This could go on forever if you never successfully down all of the messages.   
 

In simple terms this is how the email downloading (or receiving) process works.  You connect to the Internet and open Outlook Express.  Outlook Express when opened will attempt to send & receive mail for your email account. Outlook Express contacts the mail server and requests an index list of messages and then begins downloading the message(s) on that list.  After it is finished downloading ALL of the messages Outlook Express then sends a request to the server to remove the messages.  The server then deletes the messages from the email box on the server. 

If for any reason this process is interrupted and all of the messages are not downloaded, Outlook never get an opportunity to send the request to the server to remove the message.  Therefore, the next time you connect and attempt to download your email you will get the same messages again as well as any new messages that have made it to your mailbox since your last attempt.  This could theoretically go on forever if you never completed a download (POP3) session completely. 

So you might ask: What could cause a POP3 session to fail?

1. MOST COMMON CAUSE: Someone has sent you a message with a large attachment and it is taking "forever" to download so you think Outlook, your computer, or both have locked up or quit responding so you exit out of or shut down Outlook Express or your computer. The problem with this is that the message with the large attachment is message 12 of 20 so your session is not considered complete. The next time you attempt to download your email you are going to get those first 11 messages again and then of course it will appear to hang on number 12 again. This will happen again and again until you get all of the messages download.  This can be resolved by logging into webmail and deleting the message(s) with the large attachment.  The size of the message can be found in the column on the right side of the webmail screen.  After deleting this message your email will start working as you have come to expect.  Of course you can also resolve this issue by leaving the computer alone and letting it download the message.  This might take 30 or more minutes depending how large the message is.  Keep in mind that there also could be more than one message especially if you have a friend or relative who likes to send you pictures and/or large graphics.
    
2. You have an antivirus or firewall program running that is hanging Outlook Express and not allowing it to complete a POP3 session.  It is beyond the scope of this help page to troubleshoot the problems that antivirus or firewall software programs can cause.  Contact the software vendors website or technical support group for help with their software. 
   
    
3. LEAST COMMON CAUSE:  You are having a connection related issue where you can not stay connected long enough to get your email downloaded and therefore you will keep getting the same messages again and again because your POP3 session never completes fully.
    

There are several other possibilities with respect to email downloading problems but the few listed here  make up 95 plus percent of all of the support inquires we get about this issue.  Note that the large attachment problem is by far the most common.