title: Highlight cell if value is present in any cell in another column
body: <p>I want to highlight the cell in column A if it is repeated anywhere in column B. For example:</p>
  
  <pre><code>A      | B
  pack_1 | unrelated
  pack_2 | unrelated
  pack_3 | pack_1
  pack_4 | pack_1
  pack_5 | pack_3
  pack_6 | pack_3
  pack_7 | unrelated
  pack_8 | pack_2
  </code></pre>
  
  <p>In the example, <code>pack_1</code>, <code>pack_2</code> and <code>pack_3</code> should be highlighted because they are mentioned in column B.</p>
  
  <p>How can I do such a thing?</p>