How can I add a javascript alert message if checkbox is checked more than one?

My code like this :

<div class="col-md-4">
    <ul class="list-unstyled">
        <li><strong>England</strong></li>
        <li>
            <div class="checkbox">
                <label>
                    <input type="checkbox"> Chelsea
                </label>
            </div>
        </li>
        <li>
            <div class="checkbox">
                <label>
                    <input type="checkbox"> Mu
                </label>
            </div>
        </li>
        <li>
            <div class="checkbox">
                <label>
                    <input type="checkbox"> Arsenal
                </label>
            </div>
        </li>
    </ul>
</div>
<div class="col-md-4">
    <ul class="list-unstyled">
        <li><strong>Spain</strong></li>
        <li>
            <div class="checkbox">
                <label>
                    <input type="checkbox"> Madrid
                </label>
            </div>
        </li>
        <li>
            <div class="checkbox">
                <label>
                    <input type="checkbox"> Barcelona
                </label>
            </div>
        </li>
        <li>
            <div class="checkbox">
                <label>
                    <input type="checkbox"> Atletico
                </label>
            </div>
        </li>
    </ul>
</div>

Demo like this : http://ift.tt/2rkWczR

In each checkbox group, the user can only select a maximum of 1. If the user selects more than 1, I want to display alert messages

For example in england checkbox group, if user choose chelsea, then choose arsenal again, it will display alert message

How can I do it?

I had try it. But I'm still confused