ORA-01081: cannot start already-running ORACLE - shut it down first

Issue : While starting up oracle database 12.2,I am getting ORA-01081 error.

Error Message:

SQL> startup
ORA-01081: cannot start already-running ORACLE - shut it down first

Solution:
Step-1 Shutdown the database by using below command

SQL> shut immediate;
ORA-01507: database not mounted


ORACLE instance shut down.

Step-2: Then try to startup database by using startup option

SQL> startup
ORACLE instance started.

Total System Global Area 3758096384 bytes
Fixed Size                  8627392 bytes
Variable Size             872418112 bytes
Database Buffers         2868903936 bytes
Redo Buffers                8146944 bytes
Database mounted.
Database opened.