Hi Scott
You should be able to select "Do not Import Duplicates Option" while importing pst and that does not import the items that are already existing in Exchange online mailbox. This option will be visible for you after you browse and select the pst file under "options". Plese refer to below article for furtehr reference.Thanks
support.office.microsoft.com/.../Import-Outlook-items-from-an-Outlook-Data-File-pst-431a8e9a-f99f-4d5f-ae48-ded54b3440ac;ui=en-US&rs=en-US&ad=US