error while doing id query in Sql table

I am having an error when querying the id in a table, I am developing a system of friendship between users and the error is as follows:

  

PDOException: SQLSTATE [23000]: Integrity constraint violation: 1062   Duplicate du champ '0' pour la clef 'PRIMARY' in   C: \ wamp \ www \ sllap \ php \ amisade.php on line 14

See:

$is_my_friend = DB::getConn()->prepare('SELECT * FROM 'friends' WHERE (de=? AND para=?) OR (para=? AND de=?) LIMIT 1');
$is_my_friend->execute(array($idDaSessao,$idExtrangeiro,$idDaSessao,$idExtrangeiro));

if($is_my_friend->rowcount()==0){
   echo'<a class="addFriend" href="php/amisade.php?ac=addfriend&de='.$idDaSessao.'&para='.$idExtrangeiro.'">Adicionar aos amigos</a>';  
}else{
    $astatusfriend = $is_my_friend->fetch(PDO::FETCH_ASSOC);
    if($astatusfriend['status']==0){
       echo'<a class="deleteFriend" href="php/amisade.php?ac=cancel&id='.$astatusfriend['id'].'&de='.$idDaSessao.'&para='.$idExtrangeiro.'">cancelar solicitação enviada</a>';
    }
}

amisade.php page:

include('../classes/DB.class.php');

if(''<>$_GET['ac']){

   if($_GET['ac']=='addfriend'){

      $is_my_friend = DB::getConn()->prepare('SELECT * FROM 'friends' WHERE (de=? AND para=?) OR (para=? AND de=?)');
      $is_my_friend->execute(array($_GET['de'],$_GET['para'],$_GET['de'],$_GET['para']));

      if($is_my_friend->rowCount()==0){

         $addfriend = DB::getConn()->prepare('INSERT INTO 'friends' SET 'de'=?,'para'=?');
         $addfriend->execute(array($_GET['de'],$_GET['para']));

         header('Location: ../profile.php?uid='.$_GET['para']);
      }
   }
}

if($_GET['ac']=='cancel'){

   $is_my_friend = DB::getConn()->prepare('SELECT * FROM 'friends' WHERE (de=? AND para=?)OR(para=? AND de=?) LIMIT 1');
   $is_my_friend->execute(array($_GET['de'],$_GET['para'],$_GET['de'],$_GET['para']));

   if($is_my_friend->rowCount()==1){

      $addfriend = DB::getConn()->prepare('DELETE FROM 'friends' WHERE 'id'=?');
      $addfriend->execute(array($_GET['id']));

      header('Location: ../profile.php?uid='.$_GET['para']);
   }
}