This sounds similar to the problem in this post:
https://www-secure.symantec.com/connect/forums/backup-policy-firing-multiple-time
See if a cleanup of client's STREAMS* files under /usr/openv/netbackup/db/images
/client on the master server helps.
Please also log a call with Symantec support - this problem has been around since version 3.4 without any real solution... Different things seem to solve the problem (i.e. reinstall of NBU, delete/recreate policy, removing Multiple Data Streams, etc.. etc..). but this problem seems to reappear over and over.