title: not allow select same value with two dropdown
body: <p>I want to prevent the user from selecting the same value twice in this form</p>
  
  <pre><code>&lt;select name="indication_subject[]"&gt;
    &lt;option value="" selected="selected"&gt;a &lt;/option&gt;
    &lt;option&gt; Accounting&lt;/option&gt;
    &lt;option&gt; Afrikaans&lt;/option&gt;
    &lt;option&gt; Applied Information and Communication Technology&lt;/option&gt;
    &lt;option&gt; Arabic&lt;/option&gt;
    &lt;option&gt; Art and Design&lt;/option&gt;
    &lt;option&gt; Biology&lt;/option&gt;
    &lt;option&gt; Business Studies&lt;/option&gt;
  &lt;/select&gt;
  
  &lt;select name="indication_subject[]"&gt;
    &lt;option value="" selected="selected"&gt;a &lt;/option&gt;
    &lt;option&gt; Accounting&lt;/option&gt;
    &lt;option&gt; Afrikaans&lt;/option&gt;
    &lt;option&gt; Applied Information and Communication Technology&lt;/option&gt;
    &lt;option&gt; Arabic&lt;/option&gt;
    &lt;option&gt; Art and Design&lt;/option&gt;
    &lt;option&gt; Biology&lt;/option&gt;
    &lt;option&gt; Business Studies&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>