|Oracle8i Backup and Recovery Guide
To register the target database in the recovery catalog so that RMAN can access it. RMAN obtains all information it needs to register the target database from the target database itself.
If this failure occurs, you can create a second recovery catalog in another user's schema by executing create catalog (see "createCatalog") using a different user id. Then, register the database with the duplicate database identifier into the newly created recovery catalog in the new schema.
This example registers the target database, catalogs an existing datafile copy, then opens the database for use:
connect target / catalog rman/rman@rcat; startup mount; register database; catalog datafilecopy '/vobs/oracle/dbs/foo.f'; sql 'alter database open';