It's due to restability feature of fastload Say, the fastload job fails. Till the fastload failed, some number of rows was sent to the AMP's. Now if you restart FLOAD, it would start loading record from the last checkpoint and some of the consecutive rows are sent for the second time. These will be caught as duplicate rows are found after sorting of data.
This restart logic is the reason that Fastload will not load duplicate rows into a MULTISET table. It assumes they are duplicates because of this logic. Fastload support Multiset table but does not support the duplicate rows. Multiset tables are tables that allow duplicate rows. When Fastload finds the duplicate rows it discards it. Fast Load can load data into multiset table but will not load the duplicate rows.