There are a few reasons to why Outlook or any other email program will download old emails or duplicate newer ones. It could be a result of rules that were setup, corrupted or third-party software. This issue appears to only be affecting POP type accounts. To ensure that there are no duplicate emails on the server then you may browse to webmail.example.com (replace example.com with your domain name). Login using your email address and password and confirm how many of the same emails are in the inbox.
Check the rules in MS Outlook:
If you have applied multiple rules for incoming messages, Outlook may possibly create duplicates to satisfy all the applied rules. For instance, if you have set a rule to move all the emails from a sender, suppose "Thomas", to some specific folder and all the emails with subject line "Deals" to move to the other folder, in turn, you receive an email from "Thomas" with the subject line "Deals". Technically, the mail will be moved to both folders to satisfy both the rules. To prevent this, you can add "stop processing more rules" action to the rule. It will prevent further action if a rule is already applied.
In Outlook you would need to click on "File" and then "Manage Rules & Alerts". Since all rules are unique to each customer you would need to go through your existing ones to see which of them are clashing.
Antivirus and Other Securities may Create Duplicates:
When you click Send/Receive, Outlook requests mail server for the messages. Following, emails in Outlook are downloaded and connection gets closed. However, if antivirus or firewall in the system causes a failure during the process, connection does not close in a proper manner. It depicts that the emails downloaded before failure will be re-downloaded at the next Send/Receive. In this way, mailbox contains duplicate emails. To prevent such situation, you can change security settings so that the software does not interfere with Outlook. You would need to consult your administrator regarding these changes if you are not sure on how to do it yourself.
If the mail program is corrupted then it might download the emails over and over. If this is the case then you may need to reinstall Outlook or use the Microsoft repair tool to resolve the issue. Please consult your system administrator before proceeding with this.
Leaving a copy on the mail server:
If the mail program is set to leave a copy of the email on the server then there are times where it will download the same email again if it did not register that the email has already been downloaded. A workaround is to uncheck that option:
- Click on File and then Account settings
Select the account you wish to make changes to and click on "Change"
Click on More Settings on the new page that pops up, click on the Advanced tab and uncheck the "Leave a copy of messages on the server".
The mail program should not be able to download the email from the server if the copy is no longer there.