ORA-27100: shared memory realm already exists

You may get this error after restarting the database with improper memory management of SGA in SPFILE or from a Improper shutdown .

Scenario 
*******
startup nomount
 
ORA-27100: shared memory realm already exists

SQL> shutdown immediate
 
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist

Following Steps will help to overcome this issue .
          

           1)Rename existing SPFILE in the Database folder of Oracle Home.
           2)Restart the Service .
           3) Startup database using pfile with reverting changes .
           4)Create SPFILE from PFILE .
           5)Shutdown and Startup the Database using newly created SPFILE.