ORA-16751: failed to switchover to physical standby database and Switchover Ends with Two Physical Standby Databases

You might find yourself in a situation where your switchover fails and left your environment with two physical standby databases.
From the broker your might see the following error:
ORA-16816: incorrect database role
Solution:
!!!!!!!!!!!!!!!!! First of all you need to stop database instances on all servers.
1. Logon (as sysdba) to the instance that was your Primary database instance before the switchover.

2. Confirm the database role.

SQL> select database_role from v$database;

DATABASE_ROLE

-------------------------------

PHYSICAL STANDBY

3. Shutdown the instance.

SQL> shutdown immediate;

Oracle Instance Shutdown.

4. Mount the database.

SQL> startup mount;

Database opened.

5. Cancel the MRP process. You will receive "ORA-16136: Managed Standby Recovery not active" if it is not running, but you can ignore.

SQL> alter database recover managed standby database cancel;

6. Terminate the current switchover to Standby that never completed fully.

SQL> alter database recover managed standby database finish;

Database altered

7. Now switchover to Primary.

SQL> alter database commit to switchover to primary with session shutdown;

Database altered

8. Open the database.

SQL> alter database open;

Database altered.

9. Confirm the database role.

SQL> select database_role from v$database;

DATABASE_ROLE

------------------------------

PRIMARY

Additional Steps
When attempting to open the Primary Database you may suffer the following error:

SQL> alter database open

*

ERROR at line 1:

ORA-16649: possible failover to another database prevents this database being opened

In this case, before you can open the database, you must disable Data Guard Broker as follows:

 SQL> alter system set dg_broker_start=false scope=both;

 System altered.

SQL> alter database open;

 Database altered.

Now re-install Data Guard Broker.

ORA-16854: apply lag could not be determined in DGMGRL

On one of our Data Guard Architecure appeared below error in DGMGRL, when checked the configuration status but the primary and standby databases are fine with no delay and in syncronize.

ORA-16854: apply lag could not be determined

DGMGRL> show database verbose testdbst;

Database - testdbst

  Role:               PHYSICAL STANDBY
  Intended State:     APPLY-ON
  Transport Lag:      0 seconds (computed 1 second ago)
  Apply Lag:          (unknown)
  Average Apply Rate: (unknown)
  Active Apply Rate:  (unknown)
  Maximum Apply Rate: (unknown)
  Real Time Query:    ON
  Instance(s):
    testdbst

  Properties:
    DGConnectIdentifier             = 'testdbstby'
    ObserverConnectIdentifier       = ''
    LogXptMode                      = 'ASYNC'
    RedoRoutes                      = ''
    DelayMins                       = '0'
    Binding                         = 'OPTIONAL'
    MaxFailure                      = '0'
    MaxConnections                  = '1'
    ReopenSecs                      = '300'
    NetTimeout                      = '30'
    RedoCompression                 = 'DISABLE'
    LogShipping                     = 'ON'
    PreferredApplyInstance          = ''
    ApplyInstanceTimeout            = '0'
    ApplyLagThreshold               = '30'
    TransportLagThreshold           = '30'
    TransportDisconnectedThreshold  = '30'
    ApplyParallel                   = 'AUTO'
    ApplyInstances                  = '0'
    StandbyFileManagement           = 'AUTO'
    ArchiveLagTarget                = '900'
    LogArchiveMaxProcesses          = '4'
    LogArchiveMinSucceedDest        = '1'
    DataGuardSyncLatency            = '0'
    DbFileNameConvert               = '+DATA1/testdb, +DATA2/testdbst, +RECO1/testdb, +REC2/testdbst'
    LogFileNameConvert              = '+DATA1/testdb, +DATA2/testdbst, +RECO1/testdb, +RECO2/testdbst'
    FastStartFailoverTarget         = ''
    InconsistentProperties          = '(monitor)'
    InconsistentLogXptProps         = '(monitor)'
    SendQEntries                    = '(monitor)'
    LogXptStatus                    = '(monitor)'
    RecvQEntries                    = '(monitor)'
    PreferredObserverHosts          = ''
    StaticConnectIdentifier         = '(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=testdbst-dg-vip)(PORT=1522))(CONNECT_DATA=(SERVICE_NAME=testdbst)(SERVER=DEDICATED)))'
    StandbyArchiveLocation          = 'USE_DB_RECOVERY_FILE_DEST'
    AlternateLocation               = ''
    LogArchiveTrace                 = '0'
    LogArchiveFormat                = '%t_%s_%r.dbf'
    TopWaitEvents                   = '(monitor)'

  Log file locations:
    Alert log               : /u01/app/oracle/diag/rdbms/testdbst/testdbst/trace/alert_testdbst.log
    Data Guard Broker log   : /u01/app/oracle/diag/rdbms/testdbst/testdbst/trace/drctestdbst.log

Database Status:
SUCCESS

Solution: 

1) Connect to the standby database and check if MRP process is still running If MRP process is still running, cancell it with command:

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

and start MRP process with command:

SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE DISCONNECT FROM SESSION;

2) if above Solution will not work try to just restart standby database. 

3) If there is still prolem with warning "about apply lag could not be determined" you can disable lag computation with next command in DGMGRL

DGMGRL> edit database 'testdbst' set property 'ApplyLagThreshold'=0;

This definitely will solve your problem.