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.