title: count items in each category
body: <p>I've the Following Table Schema for table Items</p>
  
  <pre><code>id | item_id | category_id
  </code></pre>
  
  <p>I've sample data in this table</p>
  
  <pre><code>id  | item_id | category_id
  
  1   | 1       | 1
  2   | 1       | 2
  3   | 2       | 1
  4   | 2       | 2
  5   | 3       | 1
  6   | 1       | 1
  </code></pre>
  
  <p>Requirement: I need to count all those items which are repeating in each category_id. In the above scenario item_id '1' is repeating in category '1'. I need to count the invalid or repeating item_id against each category_id.</p>