If you're receiving duplicate emails, start at the top of the list below and work your way down. We’ve listed the most common cause first and worked our way down to the least common cause. If something jumps out at you, by all means go there first.
If you have a large amount of mail, store copies of it on the server and have Outlook check for new mail every few minutes, there's a good chance you will get duplicate email. Outlook will never complete a check mail pass before the next one starts, this prevents Outlook from marking the mail as downloaded. Try extending the time interval that Outlook checks for mail:
Click File > Options > Advanced.
Scroll down to the Send/Receive section and click Send/Receive.
Set the time interval for "Schedule an automatic send/receive every" XX Minutes to 15 minutes or more. A very large amount of mail on the server may require more time (Screen shot for this step is listed below).
Having duplicate email accounts can also result in receiving a message more than once. If you couple that with a large amount of mail on the server and the check mail time interval set to short (The issue listed above), it could become unmanageable. To check for duplicate mail accounts:
Click File -> Info -> Account Settings.
Click Account Settings from the dropdown menu.
If you see more than one account (Especially two accounts with the same name and type) (Screen shot for this step is listed below):
Highlight them one at a time and use Change to look at the account information (Screen shot for this step is listed below).
If both accounts have the same information, Remove one of them.
Multiple rules can conflict with each other, especially the Move to Folder rule if it's not configured right. Say you have these two rules:
When a message arrives from John Smith, move it to the Mail from John folder
When a message arrives with the word movie in the subject line , move it to the Movies folder
If John sends you an email with the word movie in the Subject line, Outlook will move the message to both the John Smith and Movies folders. Use Manage Rules & Alerts to edit the John Smith rule and add a "stop processing more rules" on the end of it. Obviously we can't cover all the scenarios involved with the rules you might have in this article, but you can learn all about managing and modifying Rules by reading:
When your Anti-Virus software is set to integrate with Outlook, it will "intercept" the Send/Receive request from Outlook. The Anti-Virus software becomes responsible for setting up and closing the connection properly, then telling Outlook. If the connection to the server isn’t closed properly, Outlook will not flag mail as received and you'll get duplicates.
See the documentation for your Anti-Virus software on how to disable the email protection. Once it's disabled, let Outlook go through 2 to 3 full Send/Receive cycles. If you no longer receive duplicate emails, check to see if the Anti-Virus software is configured correctly. (The company that makes your Anti-Virus software will have a Support Desk if you need any assistance.)
3rd Party Firewalls
A 3rd party Firewall that is too strictly configured could close the connection prematurely so Outlook will never have the chance to update its list of received emails. Disable your Firewall and wait 2 to 3 Send/Receive cycles. If you got get any duplicates you’ll know that your Firewall isn’t properly configured. See the documentation of your Firewall software on how to configure your firewall for e-mail retrieval with Outlook. (The company that makes your Firewall software will have a Support Desk if you need any assistance.)