Migrate Private Files: FILE_EXISTS_REUSE still duplicate files
This solved it for me.  The documentation at this link has been updated to show that you need to use the class prefix:

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

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