下拉年份显示去年两次

Question

我正在创建一个下拉按钮，该按钮可让您从选项列表中选择年份，以便对数据表进行排序。代码（转载如下）不断重复去年，我似乎无法弄清楚为什么这样做。我正在使用PHP从MySql数据库中提取数据。

      <div class="dropdown">
    <button onclick="myFunction()" class="dropbtn">Select Year</button>
    <div id="myDropdown" class="dropdown-content">
      <?php
         $year = array();

       while($subject = mysqli_fetch_assoc($subject_set)) {
           $i=0;
          $exists = 0;
          $n = count($year);
           while($i < $n){
            if ($subject['year'] == $year[$i]){
              $exists = 1;
            }
            $i++;
          }
          if($exists == 0){
            $year[$n] = $subject['year'];
            echo "<a href=\"index.php?year={$subject['year']}\"> {$subject['year']} </a>";
      }
     }
     $subject_set = find_all_subjects();
      ?>

    </div>
  </div>