No, NetBackup AFAIK is not doubling the backup data stored, and from what I am seeing this is certainly not the case.
You are assuming a image-level backup is taken of the VM snapshot, and thereafter an file-level (full-backup) is taken and stored.
Client Schedule and Incremental Backups: NetBackup 6.5.2 introduces file level incremental backups
based on vmdk-based granular file restore (Windows only) full backups. This is made possible because
NetBackup is uniquely able to index, search for, and restore individual files from a vmdk backup. Using
this vmdk backup as the basis for a full backup, NetBackup can compare the files that exist at the time of
this full backup and perform subsequent, low impact file-level incremental backups. This process can
significantly reduce backup times, I/O impact on the ESX datastore and disk/tape storage—all without
limiting restore options. Regardless of whether a full or incremental backup has been performed, any
version of any file that existed at backup time can be restored. For more information related to this
configuration, see section 6.0.
This was taken from the 2009 white paper
http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_veritas_netBackup_6.5_vmware_1-2009.pdf
If you are interested in more technical aspects check out the links on this page from some VMWorld articles which are quite good.
http://vsphere-land.com/generalvi3-links/backup-links.html
