oracle sql: How to not select select duplicate records from table
You can use NOT IN:

    SELECT DISTINCT t1.ORDER_NUMBER, t1.FLAG
    FROM TABLE t1
    WHERE t1.FLAG='true' AND t1.ORDER_NUMBER NOT IN 
    (SELECT t2.ORDER_NUMBER FROM TABLE t2
    WHERE t2.FLAG='false' OR t2.FLAG IS NULL)


If you don't care about nulls (that is you only want to exclude order numbers that explicitly have a false flag) just remove the "OR t2.FLAG IS NULL" condition. Also this query will return orders where possibly more than one records with flag 'true' exist (as long as there are no records with flag 'false' or null)