Maybe you start with reason, why you want those duplicate rows to be marked.

Do you want to delete abundant ones (a reasonable action IMHO)? Then a logical solution will be ODBC query with DISTINCT clause to get all unique entries into a separate table (the amount of rows in original table taken into account, a separate workbook will be reasonable solution). After this you can scrap the original one, or delete the sheet with original table and then move the new table into old workbook.