ORA-01430

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)

  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>