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A catalog storing backups with two databases with the same DBID would be a conflict. So a clone of a database with the same DBID would conflict with the catalog because without a unique DBID the catalog would have no way of knowing which database is which. If the catalog allowed duplicate DBID's restore/recover would be impossible as the catalog would not know which database it was working with.
The control file of the clone with the same DBID will allow a backup because the control file is completely separate from the database it was cloned from.
Thanks for your reply mseberg.
The control file of the clone with the same DBID will allow a backup because the control file is completely separate from the database it was cloned from.I am not sure I follow. Doesn't the controlfile store the DBID as well? Doesn't the DBID get reset when we clone/duplicate a database from one db to another?
Think of the catalog database as global and the control file of the clone database as local. Since the control file is local to the clone database it can store any information it wants without conflict.
The control file of the clone database is private to that database, so the DBID can be the same without conflict.
The catalog database is not private so a duplicate DBID is a conflict. ( Public to the other databases in your system )
Does that help?