rather than trying to duplicate manually, since you are using a disk backup, set the OPP of the media set that you are using so that the files are protected for as long as you wish. Also, from Tools\Options\Media Management select the option to "Use Overwriteable Media in the target media set before scratch media:"
For example of you run jobs daily, and want to keep data for a week, set the OPP of the media set to 6 days and make your job OVERWRITE. the first six times the job runs, BackupExec will create a new set of BKF files. The seventh time it runs, it will reuse the oldest set of BKF files