ORA-01810: format code appears twice

An uncommon error as this may keep you wondering. Well there is a common practice of defining both Month and Minutes as MM and this is what usually causes this error. Below is a typical example.

SQL> ALTER SESSION SET NLS_DATE_FORMAT='dd/mm/yyyy hh24:mm:ss';
ERROR:
ORA-01810: format code appears twice

Change the date format accordingly you resolve the issue

SQL> ALTER SESSION SET NLS_DATE_FORMAT='dd/mm/yyyy hh24:mi:ss';