ORA-00301: error in adding log file 'stdby02.log' - file cannot be created

SQL> alter database add standby logfile '/u02/oracle/oradata/stdby01.log' size 512M;

Database altered.

SQL> alter database add standby logfile '/u02/oracle/oradata/stdby02.log' size 512M;

Database altered.

SQL> alter database drop standby logfile '/u02/oracle/oradata/stdby01.log';

Database altered.

SQL> SELECT GROUP#,THREAD#,SEQUENCE#,ARCHIVED,STATUS FROM V$STANDBY_LOG;

GROUP#    THREAD#  SEQUENCE# ARC STATU
------ ---------- ---------- --- -----
     6          0          0 YES UNASS
                                 IGNED


SQL> alter database drop standby logfile '/u02/oracle/oradata//stdby02.log';

Database altered.

SQL> SELECT GROUP#,THREAD#,SEQUENCE#,ARCHIVED,STATUS FROM V$STANDBY_LOG;

no rows selected

SQL> ALTER DATABASE ADD STANDBY LOGFILE '/u02/oracle/oradata/stdby01.log' size 512M;
ALTER DATABASE ADD STANDBY LOGFILE '/u02/oracle/oradata/stdby01.log' size 512M
*
ERROR at line 1:
ORA-00301: error in adding log file '/u02/oracle/oradata/stdby01.log' - file cannot be created
ORA-27038: created file already exists
Additional information: 1


SQL> alter database add standby logfile '/u02/oracle/oradata/stdby03.log' size 512M;

Database altered.

SQL> alter database add standby logfile '/u02/oracle/oradata/stdby04.log' size 512M;

Database altered.