In this post, you’ll learn more about the Oracle ErrorORA-01158: database string already mounted with the details on why you receive this error and the possible solution to fix it.
Oracle Error Description
ORA-01158: database string already mounted
Reason for the Error ORA-01158: database string already mounted
Another instance has a database by this name mounted.
How to fix the Error ORA-01158: database string already mounted ?
You can fix this error in Oracle by following the below steps
Find which instance is still running. Perhaps you have not lost the control files after all.