Error Code : ORA-01102
This could happen during the switchover if you did not set the
DB_UNIQUE_NAME parameter in the initialization parameter file that is used by the standby database (that is, the original primary database). If the DB_UNIQUE_NAME parameter of the standby database is not set, the standby and the primary databases both use the same mount lock and cause the ORA-01102 error during the startup of the second database.
Action:
Add
DB_UNIQUE_NAME=unique_database_name to the initialization parameter file used by the standby database, and shut down and restart the standby and primary databases.