You cannot declare the same identifier twice in the same PL/SQL unit.
If you do, an error occurs when you reference the duplicate identifier.
SQL> SQL> DECLARE-- from w ww . jav a 2 s.c om 2 id BOOLEAN; 3 id VARCHAR2(5); -- duplicate identifier 4 BEGIN 5 id := FALSE; 6 END; 7 / id := FALSE; * ERROR at line 5: ORA-06550: line 5, column 3: PLS-00371: at most one declaration for 'ID' is permitted ORA-06550: line 5, column 3: PL/SQL: Statement ignored SQL>