At certain times, you face duplication of emails on your email client like Outlook. This article will explain you with steps to troubleshoot this issue.
1. The issue you are facing regarding duplicated mails usually occurs due to some corrupt ".pst" file for your email account. I would recommend you to re-configure your email account in your Outlook mail client.Also, please configure your email account to use the IMAP protocol instead of POP3. Because, IMAP will leave messages on the server and will sync all email data.
2. Please make sure that you haven't checked the option 'Leave a copy of messages on server' is not checked. If it is checked, please refer the following steps :
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:
1) Click File > Options > Advanced.
2) Scroll down to the Send/Receive section and click Send/Receive.
3) 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.