Grazie ho risolto così

Codice PHP:
                             reset($res);
                             
$unico = array();
                             foreach (
$res as $key => $value){
                             
$codice=$value["codice"];
                             
$nrec  =$value["num_rec"];
                             
$found false;
                             
reset($unico);
                             foreach (
$unico as $unicokey => $unicovalue) {
                                      if (
$codice == $unicovalue["codice"]){
                                            
//$unicovalue["num_rec"]=$unicovalue["num_rec"]+$nrec;
                                            
$b = array ("codice"=> $codice,
"num_rec" =>$unicovalue["num_rec"]+$nrec);

                                            
$found true;
                                            break;
                                            }
                                   }
                             if (!
$found){
                                
$unico[]=array("codice"=>$codice,"num_rec"=>$nrec);
                                }
                                else
                                {

                                
$unico[$unicokey] = $b;

                                } 
Grazie a tutti ugualmente.
Paolo