In most cases, it makes sense to go with the default Replace duplicates with items imported.
Mikagami post a comment:
The Personal Folders Backup download creates backup copies of replace.me files at regular intervals, in Outlook and later versions. Microsoft Outlook Personal Folders Backup tool is an add-in that is installed on Microsoft outlook, it is installed to automate the backup process. Select Outlook Data File .pst), and select Next. Export to data file. Select the mail folder you want to back up and select Next. Email folders.