php从数据库获取数据后，如何给teachers_xm去重？

<?php
require "db.php";
$result = mysqli_query($conn,"select teachers_info.teachers_xm , left (teach.sk_nj,4)  as 'fl' FROM teachers_info ,teach WHERE
 teachers_jgh = sk_jgh GROUP BY teachers_xm,teach.sk_nj ;
");
$array= array();
  class User{
    public $teachers_xm;
    public $fl;

  }
  while($row = mysqli_fetch_array($result,MYSQLI_ASSOC)){
    $user=new User();
    $user->teachers_xm = $row['teachers_xm'];
    $user->fl = $row['fl'];
    $array[]=$user;
  }
  header('Content-Type:application/json; charset=utf-8');
  $data=json_encode($array,JSON_UNESCAPED_UNICODE);

  echo $data;

distinct 了解下