如果两个不同的用户申请相同的订单，如何将第二个用户的值重置为null

Sorry in advance if you find my English/question hard to understand.

I am making an order apply function where users with different email addresses apply for orders. For first user who apply, everything is right but when different user applies for the same order, it shows order is already applied from its account too. How can i reset the field to NULL for new user.

Also, adding 'OR $oa_rows['user_email'] != $_SESSION['user']' is giving me int 1 and no button to click.

<?php
    $oa_sql = "SELECT * FROM order_applied RIGHT JOIN users ON 1=1 WHERE user_email = '$_SESSION[user]' ";
    $oa_run = mysqli_query($conn, $oa_sql);
    while($oa_rows = mysqli_fetch_assoc($oa_run)){
        echo '<div>';
        if($oa_rows['oa_status'] == NULL || $oa_rows['oa_user_email'] != $_SESSION['user']){ 
            if($oa_rows['oa_status'] == 'cancel' || $oa_rows['oa_status'] == 'applied' && $oa_rows['oa_user_email'] == $_SESSION['user']){
              echo '<div style="display:none">
               <form method="post">
                  <input type="hidden" name="oa_status" value="applied">
                  <input class="btn btn-sm btn-info" type="submit" name="oa_status_submit" value="Apply For Order">
               </form>
              </div>'
          } else {
             echo '<div>
            <form method="post">
                <input type="hidden" name="oa_status" value="applied">
                <input class="btn btn-sm btn-info" type="submit" name="oa_status_submit" value="Apply For Order">
            </form>
              </div>';
            }
        } else {
            if($oa_rows['oa_status'] == 'applied'){ 
                echo '<a href = "order_details.php?o_id='.$rows['_id'].'&new_status=cancel" class="btn btn-sm btn-danger" >Cancel Request</a>';
            } else {
                echo '<a href="order_details.php?o_id='.$rows['_id'].'&new_status=applied" class="btn btn-sm btn-warning" >Apply For Order</a>';
            }
        }
        echo '</div>';
    }