To find duplicate customer details entered under different CUSTNO values, one would typically use methods that compare records within the same table:

C. Subquery: A subquery can be used to compare customer details in the CUSTOMERS table and identify duplicates. This subquery can look for matches in the customer details while having different CUSTNO.

E. Self Join: A self join can also be used for this purpose. This involves joining the CUSTOMERS table to itself on the columns containing customer details to identify rows that have the same information but different CUSTNO.

References:

Oracle Database SQL Language Reference 12c, particularly the sections on self joins and subqueries, as these can be utilized to find duplicate rows based on specified criteria.