One of the great new features that comes in very handy when databases get larger and larger these days is RMAN's capability to duplicate from an active database and even restart a duplicate when it fails.

Imagine yourself the problem I had lately; I used the duplicate from active database feature and had to wait for an hour or 6 before all datafiles where transferred.At the end of the process some error occurred because of the syntax. While this error was easily to solve I was afraid I had to redo the complete procedure and transfer the 2.5 TB again.
Well, 11gr2 RMAN surprised when I re-ran my command with the following output:

Using previous duplicated file +DATA/fin2prod/datafile/users.2968.719237649 for datafile 12 with checkpoint SCN of 183289288148

Using previous duplicated file +DATA/fin2prod/datafile/users.2703.719237975 for datafile 13 with checkpoint SCN of 183289295823

The documentation says this:
RMAN automatically optimizes a DUPLICATE command that is a repeat of a previously failed DUPLICATE command. The repeat DUPLICATE command notices which datafiles were successfully copied earlier and does not copy them again. This applies to all forms of duplication, whether they are backup-based (with and without a target connection) or active database duplication. The automatic optimization of the DUPLICATE command can be especially useful when a failure occurs during the duplication of very large databases.
If a DUPLICATE operation fails, you need only run the DUPLICATE again, using the same parameters contained in the original DUPLICATE command.

Please see chapter 23 of the 11g Release 2 Database Backup and Recovery User's Guide for more details.

B.w.t. be very careful with the duplicate command. A small mistake in one of the 'convert' parameters can potentially overwrite your target's controlfile without prompting !

Rene Kundersma
Technical Architect
Oracle Technology Services