This was tested on Oracle 11.2. If you try to use a column name twice in the same table, you get an ORA-01430:
SQL> create table tab1 (col1 number)
SQL> create table tab1 (col1 number)
2 /
Table created.
SQL> alter table tab1 add (col1 number)
2 /
alter table tab1 add (col1 number)
*
ERROR at line 1:
ORA-01430: column being added already exists in table
SQL>
