title: grouping/counting/matching data in R
body: <p>I have a large dataset containing several columns of different values. Have a look below:</p>
  
  <pre><code>X             Y         Z
  20:00:00     AAA    ABC123*
  20:00:00     BBB    ABC123*
  21:00:00     AAA    AEC173
  23:00:00     CCC    ABE124
  20:00:00     DDD    ABC123*
  </code></pre>
  
  <p>and so on...</p>
  
  <p>Column <code>X</code> is containing time, <code>Y</code> is three letter code and <code>Z</code> is alphanumerical code. 
  I would like <code>R</code> to highlight me only the occurrences when both <code>X</code> and <code>Z</code> column values are same, but Y value is different. Such a case would be first two columns that are marked with star and also the last one. </p>