Dupes are often firmware related, but can also be network connectivity (resubmission of a share due to lack of response in a timely manner). I know FPGAs were big culprits of duplicates, but they weren't actually *wasted* work, it was simply that there was some kind of firmware bug that caused them to report the same nonce information in certain scenarios where no solution was found when the nonce range was depleted.
My Avalon lately has been mostly clean on dupes with later firmware, generally a completely insignificant amount. It may be network connectivity, since my Avalon is on wireless across the house, with a microwave in the middle
