Migrate d2d – Problem with duplication of thumbnails
Finally found a workaorund… by deliberately giving the destination field an empty value instead of not doing anything when $fid2 is FALSE, I got rid of all the duplications :-)

if($fid2 === FALSE){
    $filepath = array();
    $this-&gt;addFieldMapping('field_thumbnail_newsite')-&gt;defaultValue($filepath);
} else {
    $filepath = Database::getConnection('default', 'legacy')-&gt;query('SELECT f.filepath FROM {files} f WHERE f.fid = :fid',array(':fid' =&gt; $fid2))-&gt;fetchAssoc();    
    $this-&gt;addFieldMapping('field_thumbnail_newsite')-&gt;defaultValue($filepath);
}