How to use "WHERE NOT EXISTS in INSERT STATEMENT" - Mailing list psycopg

cur.execute("INSERT INTO COMPANYLIST (ID, NAME, AGE, ADDRESS, SALARY) \      
        VALUES (1, 'Paul', 32, 'California', 20000.00)");

I get the error below when running the code twice.

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psycopg2.IntegrityError: duplicate key value violates unique constraint "companylist_pkey"
DETAIL:  Key (id)=(1) already exists.

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cur.execute("INSERT INTO COMPANYLIST (ID, NAME, AGE, ADDRESS, SALARY) \
        WHERE NOT EXISTS (SELECT 1 FROM table WHERE id='1')\
        VALUES (1, 'Paul', 32, 'California', 20000.00)");