duplicate column name
select e.empno,e.deptno,e.ename,d.ename from emp e,emp d where e.mgr=d.empno and e.hiredate<d.hiredate;

  EMPNO DEPTNO ENAME ENAME
---------- ---------- ---------- ----------
  7499 30 ALLEN BLAKE
  7521 30 WARD BLAKE
  7698 30 BLAKE KING
  7566 20 JONES KING
  7782 10 CLARK KING
  7369 20 SMITH FORD

6 rows selected.

SQL> create table empa as select e.empno,e.deptno,e.ename,d.ename from emp e,emp d where e.mgr=d.empno and e.hiredate<d.hiredate;
create table empa as select e.empno,e.deptno,e.ename,d.ename from emp e,emp d where e.mgr=d.empno and e.hiredate<d.hiredate
  *
ERROR at line 1:
ORA-00957: duplicate column name

------解决方案--------------------
提示比较明显了，就是列名重复。
如楼主的做法，起不同的别名就OK了