Avoid duplicate files on migrate update
This solved it for me.  

The documentation at this link was missing a mention that you need to use the class prefix - I just submitted an update to the Drupal Docs to make it more clear:

$this-&gt;addFieldMapping('preserve_files')
  -&gt;defaultValue(1);

$this-&gt;addFieldMapping('file_replace')
  -&gt;defaultValue(MigrateFile::FILE_EXISTS_REUSE);