How to do Office 365 PST Import while not allowing duplicates
The native Office 365 PST network import tool will not give you any tools to filter/remove duplicates: https://support.office.com/en-us/article/Use-network-upload-to-import-your-organization-s-PST-files-to-Office-365-103f940c-0468-4e1a-b527-cc8ad13a5ea6
Your options are limited to:
Use a third party tool to import PST: this will have the option to stop duplicates from being imported into the mailbox. Those are usually paid for and will cost you, I'm listing this in case you have a large number of users who needs their data imported.
Import using Outlook: You can use Outlook import filters to detect and stop duplicates from being imported to the mailbox. https://support.office.com/en-us/article/Import-email-contacts-and-calendar-from-an-Outlook-pst-file-431a8e9a-f99f-4d5f-ae48-ded54b3440ac?ui=en-US&rs=en-US&ad=US
Switches in the native Office 365 PST network import tool: I know this doesn't remove duplicates, but in case the above fails, you can restore a PST to a subfolder of your choosing using the "TargetRootFolder" option.
Trim the data before importing: Again, it doesn't remove duplicates, but you may choose to restore data up to a specific date, maybe continue where the data was cut off? https://support.office.com/en-us/article/Filter-data-when-importing-PST-files-to-Office-365-26af16df-34cd-4f4a-b893-bc1d2e74039e
As per https://docs.microsoft.com/en-us/microsoft-365/compliance/importing-pst-files-to-office-365?view=o365-worldwide, section "How does the PST import process handle duplicate email items?"
The PST import process checks for duplicate items and doesn't copy the items from a PST file to the mailbox or archive if a matching item exists in the target folder in the target mailbox or target archive. If you reimport the same PST file and specify a different target folder (using the TargetRootFolder property in the PST import mapping file) than the one you specified in a previous import job, all items in the PST file will be reimported.
So out-of-the-box no duplicates should be created if you re-import your original .pst file to your Microsoft 365 mailbox.