What criteria determines if the "Save and Next" option is available in the fields list of a table when needing to edit the duplicated fieldsfields.

I believe that it just goes by your default list of fields - the last field in the list does not give you an option because there is no "next field". In the fields list, try sorting the list by Field ID or Field Label first, then going back in. In my list, with no sorting applied, all fields give me "Save and Next" except Last Modified By.