How to copy files to postgresql db from a csv file without duplicate data?

i am using python code with psycopg2 to automate the process of downloading csv files from a website and copying to database but i need a way that does not allow duplicate data persistence, what would be the best way around this situation?

i am currently using this code to copy:

COPY table_name(item ,order ,cn_main,cn_second)
                FROM '{dir.get_cn_folder()}'
                DELIMITER ';'
                CSV HEADER encoding 'ISO-8859-1';