title: total number of duplicates in oracle table
body: <p>I have a table like below</p>
  
  <pre><code>SUBJECT Years MARKS
  AB      1       20
  AB      1       25
  AC      1       20
  AC      1       30
  AC      1       40
  AD      1       20
  </code></pre>
  
  <p>I only need count of duplicates (subject||Year), expected answer is 2
  and not </p>
  
  <pre><code>AB1 -- 2
  AC1 -- 3
  </code></pre>