Public Folder Sync–Duplicate Name Error

I came across this error with a client today and did not find it documented anywhere – so here it is!

When running the Public Folder sync script Sync-ModernMailPublicFolders.ps1 which is part of the process of preparing your Exchange Online environment for a public folder migration, you see the following error message:

UpdateMailEnabledPublicFolder : Active Directory operation failed on O365SERVERNAME.)365DATACENTER.PROD.OUTLOOK.COM. The
object ‘CN=PublicFolderName,OU=tenant.onmicrosoft.com,OU=Microsoft Exchange Hosted
Organizations,DC=)365DATACENTER,DC=PROD,DC=OUTLOOK,DC=COM’ already exists.
At C:\ExchangeScripts\pfToO365\Sync-ModernMailPublicFolders.ps1:746 char:9
+         UpdateMailEnabledPublicFolder $folderPair.Local $folderPair.Remote;
+         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
     + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,UpdateMailEnabledPublicFolder

This is caused because you have a user or other object in Active Directory that has the same name as the mail enabled public folder object.

In Exchange Online PowerShell if you run Get-User PublicFolderName you should not get anything back, as its a Public Folder and not a user, but where you see the above error you do get a response to Get-User (or maybe Get-Contact or any other object that is not a Public Folder. This class of object name (common name or cn) means the script can create the public folder in the cloud, but not update it on subsequent runs of the script.

The easiest fix is to rename the common name of the public folder object in Active Directory for all clashing public folders, unless you know you do not need the other object that clashes – as renaming that and letting AADConnect sync process the change is another way to resolve this.

To rename the mail public folder, in Exchange Server management shell run Set-MailPublicFolder PublicFolderName –Name NewPublicFolderName

I have changed my names to start with pf, so PublicFolderName becomes pfPublicFolderName and then the script runs without issue.