title: jQuery append if doesn't exist
body: <p>I would like to append a <code>div</code> only if it's not already there. I am trying with this but it doesn't work:</p>
  
  <pre><code>$('#method_id').on('change', function (e) {
  
      if ($(this).find("option:selected").data('method-column-id') != 1) {
          if ($('#column_message').next('div').length)
              $('#column_message')
                      .append('&lt;div id="warning_message" class="alert alert-danger"&gt;&lt;strong&gt;Warning!&lt;/strong&gt; Installed column and method do not match&lt;/div&gt;');
      } else {
          $('.form-group').find('#column_message&gt;.alert').remove();
      }
  });
  </code></pre>
  
  <p>And if I remove the second if-clause, it gets appended every time I select an option which passes first if-clause</p>
  
  <p>Here is the HTML</p>
  
  <pre><code>&lt;!-- Warning message --&gt;
  &lt;div class="form-group"&gt;
      &lt;label for="group" class="col-md-2 control-label"&gt;&lt;/label&gt;
      &lt;div id="column_message" class="col-md-6"&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>