What is meant by the RMAN Duplicate "noncurrent point in time"?
It's the moment of the last redolog archival, before some point in time during the RMAN duplicate command. You can test this easily, if you either manually provoke redolog switches (and thus subsequent archivals), or even better: write some script to provoke very frequent and unnecessary redolog switches. In which you perform some insert/update/delete, to further track things down.