Just a brief note to share with you that if you are performing an RMAN duplication over an existing instance, you'll want to delete your temporary tablespace tempfiles first. Metalink Note 374934.1 can tell you the whys and wherefores. The note also seems to suggest that the problem exists solely in Oracle 10.2.0.2.
The note suggests getting the list of datafiles with "select * from dba_temp_files;", doing a "startup force nomount" and then deleting the files via OS commands, before finally kicking off the RMAN duplicate command. I had been toying with a different method (creating a new temp2 tablespace and dropping the original with contents and datafiles), but this method seems to be cleaner. Plus it is the WORD OF METALINK.