How to do Office 365 PST Import while not allowing duplicates


We started a PST import to a specific subfolder for a group of users. The PST import failed half-way. Now we notice that the users are having the imported PST in their mailbox but only half of the data. We want to re-try the import but we don't want to overwrite the mails that have already been imported.

On-premises Exchange had a convenient parameter for this but it seems the PST Import service of Office 365 doesn't have this.

We can't delete the folders that have been created by the import because users might have used them already (to store new mail items).

Any ideas?
Jozef WooSystem EngineerAsked:
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Tom CieslikIT EngineerCommented:
In process if importing file to Outlook, there is an option to select: Do Not Import Duplicates" or Replace Duplicates with Items Imported

Jozef WooSystem EngineerAuthor Commented:
Hi Tom, thanks but I know this option. I'm talking about the PST Import service in Office 365.
Tom CieslikIT EngineerCommented:
This problem was resolved on EE before.

Bottom line is you need to import all as is then use script to remove duplicates
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Jozef WooSystem EngineerAuthor Commented:
Hi Tom, thanks for your feedback. There is one more solution:

You can trigger the import via PowerShell and use the familiar paramters, here's an example article:

The same New-MailboxImportRequest cmdlet is used (

You need a Premium Azure Subscription though.
Jozef WooSystem EngineerAuthor Commented:
There are 3 options:

- Using Outlook
- Importing them anyway and then removing the duplicates
- Using the New-MailboxImportRequest while importing from Premium Azure Storage.
Jozef WooSystem EngineerAuthor Commented:
Actually.... the real answer to this question is:

Office 365 PST Import does NOT create duplicates !!! (despite what the Office 365 Support team told me!!!) I tested it myself.
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