ORA-19909: datafile 1 belongs to an orphan incarnation

One database might have multiple incarnations.

The following errors occurred in a standby database alert log :

MRP0: Detected orphaned datafiles!
ORA-19909: datafile 1 belongs to an orphan incarnation
ORA-01110: data file 1: '+DATA/PROD/datafile/system.238.875096219

Check standby database incarnation :

RMAN> list incarnation;

using target database control file instead of recovery catalog

List of Database Incarnations
DB Key Inc Key DB Name DB ID    STATUS Reset SCN    Reset Time
------ ------- ------ ---------- ----- ------------ ----------------
1      1       PROD   3779924890 PARENT 1           08-01-2015 10:00
2      2       PROD   3779924890 CURRENT 2620491565 01-02-2016 12:01

Reset database to old incarnation :

RMAN> reset database to incarnation 1; 

database reset to incarnation 1 


RMAN> list incarnation;

using target database control file instead of recovery catalog

List of Database Incarnations
DB Key  Inc Key DB Name DB ID      STATUS  Reset      SCN Reset Time
------- ------- ------- ---------- ------- ---------- ---------------
1       1       PROD    3779924890 CURRENT 1          08-01-2015 10:00
2       2       PROD    3779924890 ORPHAN  2620491565 01-02-2016 12:01

Then everything is working fine.