Working with Recovery Catalog.

Registering Multiple Databases in a Recovery Catalog
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You can register multiple databases in a recovery catalog; that means you can keep multiple database repository information in a single recovery catalog. But one restriction is each database DBID must be different as RMAN distinguish one database from another by DBID.

So whenever you just copy one database with user managed copy or by RMAN restore and recover then both database DBID is same. So they can't be register in same recovery catalog. In that case if you want to do so you have to change DBID. The method of changing DBID is described here Change DBID.

Unregistering a Target Database from the Recovery Catalog
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To unregister a target database from the recovery catalog, just follow the procedure.
1)Connect to Recovery Catalog database.

-bash-3.00$ rman target / CATALOG catalog_user/catalog_pwd@saturn:1521/ARJU.SATURN.ARJUBD.COM
Recovery Manager: Release 10.2.0.1.0 - Production on Wed May 7 23:53:51 2008
Copyright (c) 1982, 2005, Oracle. All rights reserved.
connected to target database: DBASE (DBID=1509380669)
connected to recovery catalog database

2)issue UNREGISTER DATABASE command.

RMAN> UNREGISTER DATABASE;
database name is "DBASE" and DBID is 1509380669

Do you really want to unregister the database (enter YES or NO)? YES
database unregistered from the recovery catalog

But remember when a database is unregistered from the recovery catalog, all RMAN repository records in the recovery catalog are lost. The database can be registered again, but the recovery catalog records for that database are then based on the contents of the control file at the
time of re-registration. Records older than the CONTROLFILE_RECORD_KEEP_TIME setting in the target database control file are lost.

Resynchronizing the Recovery Catalog
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RMAN performs resynchronizations automatically as needed when you execute certain commands, including BACKUP. If you want to manually resynchronize issue RESYNC CATALOG command. In case of resynchronization RMAN compares the recovery catalog to either the current control file of the target database or a backup control file and updates the recovery catalog with information that is missing or changed.

RMAN> RESYNC CATALOG;
starting full resync of recovery catalog
full resync complete