Here are the Simple steps to recover data file from stand by/ backup server.
1) On standby/ backup database, copy datafile from ASM to a file system if using ASM:
RMAN> backup as copy datafile 71 format '/tmp/df71.dbf';
2) transfer the datafile copy from the standby to the primary host using scp.
On primary database
3) Place the datafile to recover offline.
SQL> alter database datafile 71 offline;
4) catalog this datafile copy:
RMAN> catalog datafilecopy '/tmp/df71.dbf';
5) Confirm that datafile exists:
RMAN> list copy of datafile 71;
6) Restore the datafile:
RMAN> restore datafile 71;
7) Recover the datafile:
RMAN> recover datafile 71;
8) Place the datafile online:
SQL> alter database datafile 71 online;
