问防止重复点击按钮时再次加载标签图片

EN

<section id="vertical_tab_nav">
    <ul>
    <br><br><br>
       <li id="#btnPro"><a href="index.html">Redes afectivas</a></li>
       <li id="#btnPro" class="selected"><a href="index.html">Hooke</a></li>
       <li id="#btnPro"><a href="index.html">Flizen</a></li>
    </ul>
    <div>
       <article class="uno"></article>
       <article class="dos"></article>
       <article class="tres"></article>
    </div>
</section>

#vertical_tab_nav div article{
display: none;
margin: 0px;
color: #555;
transition: background 2s ease-out;
}

#vertical_tab_nav div .uno{
background-image: url(../img/solucion1.png);
background-repeat: no-repeat;
background-size: 90%;
margin: 0px;
margin-top: 0px;
color: #555;
min-height: 500px;
background-position: top;
margin-top: 10px;
}