A way to remove the duplicate records

First you need to query the database to find all duplicate rows, and then you need to run a statement to delete one of each duplicate record that is found

DECLARE
  CURSOR emp_cur IS
  SELECT *
  FROM   employees
  ORDER BY employee_id;
  emp_count         number := 0;
  total_count         number := 0;
BEGIN
  DBMS_OUTPUT.PUT_LINE('You will see each duplicated employee listed more ');
  DBMS_OUTPUT.PUT_LINE('than once in the list below.  This will allow you to ');
  DBMS_OUTPUT.PUT_LINE('review the list and ensure that indeed...there are more ');
  DBMS_OUTPUT.PUT_LINE('than one of these employee records in the table.');
  DBMS_OUTPUT.PUT_LINE('Duplicated Employees: ');
-- Loop through each player in the table
  FOR emp_rec IN  emp_cur LOOP
-- Select the number of records in the table that have the same ID as the current record
     SELECT count(*)
     INTO emp_count
     FROM employees
     WHERE employee_id = emp_rec.employee_id; 
-- If the count is greater than one then a duplicate has been found, so print it out.
     IF emp_count > 1 THEN
        DBMS_OUTPUT.PUT_LINE(emp_rec.employee_id || ' - ' || emp_rec.first_name ||
                   ' ' || emp_rec.last_name || ' - ' || emp_count);
                  total_count := total_count + 1;
     END IF;

  END LOOP;
END;


Next, you need to delete the duplicated rows that have been found. The following DELETE statement
will ensure that one of the duplicates is removed:

DELETE FROM employees A WHERE ROWID > (
SELECT min(rowid) FROM employees B
WHERE A.employee_id = B.employee_id);