This was tested on Oracle 11.2. If you define a column as NOT NULL then try to modify it to NOT NULL again, you get an ORA-01442:
SQL> create table tab1 (col1 number not null)
2 /
Table created.
SQL> alter table tab1 modify (col1 number not null)
2 /
alter table tab1 modify (col1 number not null)
*
ERROR at line 1:
ORA-01442: column to be modified to NOT NULL is
already NOT NULL
SQL>