We came across something in RMAN while working on a duplicate database. The ‘skip tablespace’ command when doing an RMAN duplicate database will work for self-contained tablespaces, but not for a set of self-contained tablespaces. In other words, if each tablespace can be individually dropped, its fine, but where the indexes are placed separately from the tables, this won’t work for the duplicate.
Oracle calls it “not a bug”. read more on Metalink.