Datapump across network link exhibits duplicate successive block reads

In the process of migrating a table across a platform boundary (AIX -> Linux) which contained a couple of LOB columns (the columns are XMLTYPEs) we ran into a problem which lead me to trace the sessions involved in reading the data.  It was at this point that I noticed that for the LOB columns, the trace reports that each block is being read twice in succession.  Here is an excerpt of the trace output:
WAIT #2: nam='direct path read' ela= 559 file number=8 first dba=196794 block cnt=1 obj#=10522 tim=31109815875816
WAIT #2: nam='direct path read' ela= 300 file number=8 first dba=196794 block cnt=1 obj#=10522 tim=31109815876338