添加数据时如何使选定的预约ID不再出现？

I have a program like the one below, how do I make the selected reservation id not reappear when adding data?

<div class="form-group">
    <label for="reservation_id" class="col-md-4 control-label">Reservation</label>
    <div class="col-md-8">
        <select name="reservation_id" class="form-control">
            <option value="">select reservation</option>
            <?php 
            foreach($all_reservations as $reservation)
            {
                $selected = ($reservation['id'] == $this->input->post('reservation_id')) ? ' selected="selected"' : "";

                echo '<option value="'.$reservation['id'].'" '.$selected.'>'.$reservation['id'].'</option>';
            }
            ?>
        </select>
    </div>
</div>