Duplicate Folder Outlook Sync Issue

Question

Hello,

i need your help.

I'm an IT-Admin in a small company and my boss has a strange issue.

Sometimes if he is creating a new Folder in Outlook it gets duplicated. The strange thing is we do not know when it happens to reproduce. If he is creating it two folders with the same name will appear.

In outlook web there is only one folder. So it has to be something with the Sync because i already wiped his pc completely so no local issue from pc.

I want to see some logs somewhere but i don't know where to look why a second folder is created.

Hope someone can help me.

Answer 1

Dear Andreas,

Thank you for posting your question on the Microsoft Q&A forum.

I’m sorry to hear that Outlook sometimes creates duplicate folders when a new folder is created. Since the issue only appears in the desktop client of Microsoft Outlook, it is likely related to sync or profile configuration. You can follow the steps below:

1/ Confirm cached exchange mode status

In desktop Outlook, go to File > Account Settings > Account Settings > Double-click the account > Select Change

Check whether Cached Exchange Mode is enabled

If it is enabled:

• Temporarily disable Cached Exchange Mode
• Restart Outlook
• Test creating a new folder again

2/ Create a new mail profile

• Creating a new Outlook profile. For guidance, please refer to this article: https://support.microsoft.com/en-us/office/create-an-outlook-profile-f544c1ba-3352-4b3b-be0b-8d42a5…
• Set the new profile as the default
• Remove the old profile after confirming the new one works properly

3/ Check for shared mailbox or delegate sync conflicts

If your boss:

• Has delegated mailbox access
• Uses shared mailboxes
• Has the mailbox open on multiple devices simultaneously

Folder creation from multiple endpoints may occasionally trigger duplicate entries during synchronization.

Please verify:

• Does the issue occur when Outlook is open on another device?
• Is Outlook running on another device at the same time?

4/ Disable Add-ins

Add-ins can sometimes interfere with folder synchronization.

• In Outlook, go to File > Options > Add-ins
• Select Manage: COM Add-ins > Click Go
• Disable all add-ins
• Restart Outlook
• Test again

5/ Check Outlook version (Classic and New)

If using the New Outlook for Windows, please switch temporarily to the Classic version and test again, as folder sync behavior differs between the two versions.

6/ Repair/ rebuild the OST File

Please repair the Outlook data file as follows: Repair Outlook Data Files (.pst and .ost) - Microsoft Support

If the issue persists, you can rebuild the OST file (local mailbox cache):

• Close Outlook
• Navigate to:

C:\Users%username%\AppData\Local\Microsoft\Outlook

• Delete the corresponding .ost file
• Reopen Outlook (the OST file will automatically rebuild and re-sync from the server)
• Test folder creation again

For reference: https://learn.microsoft.com/en-us/troubleshoot/outlook/data-files/errors-have-been-detected-in-.ost…

I hope this information helps resolve the issue. If you have any additional questions or need further clarification, please feel free to let me know.

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