RMAN ASM Duplication Orphan Files?

I use the following script to duplicate a target to an auxiliary instance. This works fine once.

However if I run it a second time it will create a new set of datafiles leaving the original files in place taking up space.

Is there something that I can put in the script that will stop this happening.

I know I could probably put a "set newname for datafile" clause for each datafile in the target database, but this makes the script over complicated and I would like to keep it as generic as possible.

DUPLICATE TARGET DATABASE TO dupdb
until time "TO_DATE('07/06/2011 07:25','DD/MM/YYYY HH24:MI')"
LOGFILE GROUP 1 ('+DATA') SIZE 50M,
GROUP 2 ('+DATA') SIZE 50M,
GROUP 3 ('+DATA') SIZE 50M,
GROUP 4 ('+DATA') SIZE 50M;