It will merge based on the unique identifier which is likely to be an email address.
A simple way would be to do an export of your contacts before the import and then compare that to the import csv and look for duplicate email addresses.
I am not sure it can be done inside of the Mautic UI.
I’ve got easier solution. When you do import add some tag, as “import20221013”. After import type in contact search window: “tag:import20221013”, and order contact by ID. The oldest were merged.
I stumbled across the solution tonight in Mautic. When you do an import, Mautic will indicate which contacts were inserted and which contacts were updated (merged).