<div class="odform">
  <form action="#" method="post">

    <div class="input-group">
      <label for="amount_id">金额</label>
      <input type="text" name="amount" id="amount_id"
          placeholder="单位是元，精确到分,如:12.66" pattern="^([1-9]+(\.[0-9]{2})?|0\.[1-9][0-9]|0\.0[1-9])$">
      <input type="hidden" name="code" value="<?php $code = mt_rand(0, 100000); echo $code; ?>">
    </div>

    <div class="input-group">
      <span id="available">当前余额:<?php echo $amount_available; ?>元</span>
    </div>

    <button name="confirm">确认</button>
  </form>
</div>

if(isset($_POST['confirm'])) {
  //解决重复调用两次的问题
  if ($_POST['code'] == $_SESSION['code'])
  {
    exit;
  }
  else
  {
    $_SESSION['code'] = $_POST['code'];
  }

  //continue....
}