SQL & PL/SQL :: Update Job-id Of Employees Table Of HR Schema?

I am trying to update of job_id column of employees table for employee number 205 two times one after another. First time job_id column of employees table for employee number 205 is updated with new job_id. But second time job_id column of employees for employee number 205 table can not be updated. Oracle returns the following errors

HR:orcl > update employees set JOB_ID='AC_MGR' where employee_id=205;
update employees set JOB_ID='AC_MGR' where employee_id=205
*
ERROR at line 1:
ORA-00001: unique constraint (HR.JHIST_EMP_ID_ST_DATE_PK) violated
ORA-06512: at "HR.ADD_JOB_HISTORY", line 10
ORA-06512: at "HR.UPDATE_JOB_HISTORY", line 2
ORA-04088: error during execution of trigger 'HR.UPDATE_JOB_HISTORY'

As there is composite primary key using employee_id and start_date column.how to update same employee job_id twice.AS we can see from job_history table, the record for employee 200 is as follow

EMPLOYEE_ID START_DATE END_DATE JOB_ID DEPARTMENT_ID
----------- ----------- ---------- ---------- -------------
200 17-SEP-1987 17-JUN-1993 AD_ASST 90
200 01-JUL-1994 31-DEC-1998 AC_ACCOUNT 90

So how can i do the same for employee 205 without changing hire_date after first job_id update. Since for every update of job_id fires trigger. To insert row in job_history table employee id and start_date must be unique each time. HERE hire_date of employees table is used as start_date of job_id table. how it was possible for employee id 200 to change job_id twice?