Why does "VALIDATE RECOVERY FILES" create copies of the files it validates?

I was experimenting with different validation options, and ran VALIDATE RECOVERY FILES in RMan.  Oddly, this appears to create a copy of each backup piece as it is validated.  What is the purpose of this (or is it perhaps a bug)?  This behavior will prevent me from running this particular validation command for some databases, since I don't have enough excess disk space to hold a second copy of every backup piece.
The format mask we are using for the output files is "%d-I_%I-S_%s-P_%p-U_%u-T_%T_%t".  The only difference in the naming of the copy created by the validation command is the date, corresponding to %T in the format mask.