Hello,
Thank you for posting in Q&A forum.
This issue can be caused by a number of different reasons, such as incorrectly configured message forwarding or incorrectly configured Outlook rules.
To resolve this issue and prevent duplicate items from appearing in Outlook, you need to find the specific cause of duplicate items in your Outlook account. Please refer to the following link:
18 Reasons Why Outlook Duplicates Emails, Contacts, Tasks, and Calendar Entries | Outlook Freeware
After checking and fixing the errors, you can resume normal Outlook operations and resolve duplicate emails.
I hope the information above is helpful.
Best Regards,
Yanhong Liu
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