Oracle Solution for Error ora-16002

ORA-16002: database already open for read-write access by another instance

What triggered the Error:

The database has been opened for read-write access by another instance, and cannot be opened for read-only access by this instance.

What should we do to fix it:

This instance must be opened for read-only access, or all other instances must first be shut down and re-opened for read-write access.