You use the IS A SET operator to check if the elements in a nested table are distinct.
SQL> CREATE OR REPLACE PROCEDURE is_a_set_example AS
2 TYPE nestedTableType IS TABLE OF VARCHAR2(10);
3 myTable1 nestedTableType;
4 result BOOLEAN;
5 BEGIN
6 myTable1 := nestedTableType('F', 'G', 'S', 'G');
7 result := myTable1 IS A SET;
8 IF result THEN
9 DBMS_OUTPUT.PUT_LINE('Elements are all unique');
10 ELSE
11 DBMS_OUTPUT.PUT_LINE('Elements contain duplicates');
12 END IF;
13 END is_a_set_example;
14 /
Procedure created.
SQL> CALL is_a_set_example();
Elements contain duplicates
Call completed.
SQL>