title: Find a list of users with duplicate transactions
body: <p>I have the following table:</p>
  
  <pre><code>Transaction_ID  int(9)  NO  PRI NULL    auto_increment
  Datetime    date    NO      NULL     
  Giver_ID    int(9)  NO      NULL     
  Recipient_ID    int(9)  NO      NULL     
  Points  int(4)  NO      NULL     
  Category_ID int(3)  NO      NULL     
  Reason  text    NO      NULL     
  </code></pre>
  
  <p>Apparently I've managed to award some of our students double points for "Behaviour Week".</p>
  
  <p>I would like a list of all the rows in this <code>transactions</code> table where a single user (<code>Recipient_ID</code>) has more than one transaction with <code>Category_ID = 16</code>.</p>
  
  <p><strong>Each <code>Recipient ID</code> should only have ONE transaction with <code>Category_ID = 16</code>.</strong></p>
  
  <p>For example, if I had...</p>
  
  <pre><code>54784   2012-02-01  138573  137444  100 16  You have done very well in Behaviour Week! Because...
  55040   2012-02-01  138573  132629  100 16  You have done very well in Behaviour Week! Because...
  </code></pre>
  
  <p>That's two separate users, which is fine.</p>
  
  <p>However, if I find...</p>
  
  <pre><code>54784   2012-02-01  138573  137444  100 16  You have done very well in Behaviour Week! Because...
  55040   2012-02-01  138573  137444  100 16  You have done very well in Behaviour Week! Because...
  </code></pre>
  
  <p>Then I've made a mistake and I need to delete one of those rows.</p>
  
  <p>Could someone please suggest a query which will list all of these potentially incorrect transactions? I'd prefer for them just to be "SELECT" so that I can manually check rather than blindly DELETE-ing.</p>