Backups/Restores work, Duplications fail

Master and Media Server:

NB 7.5.0.4

RHEL 6.3

Data Domain DD880 (VTL)

STK L700e w/ IBM LTO3 drives

When we do a backup or restore to VTL or physical tape, both procedures work fine.  When we try to do a duplication, the dupe hangs or may take 3-4 days to dupe what was a 30 minute backup.  This occurs with dupes from  VTL->VTL, VTL->Physical Tape, Physical ->Physical.  Also doesn't matter whether we use Vault, SLP, or dupe from the catalog image manually, the same thing happens.

Any Ideas?  We have been working with a Symantec Tech for several months on this, thought I would finally put the question out here.

Thanks in advance for any replies