title: Laravel excel validate two columns' combination is not duplicate
body: <p>I have a requirement wherein I need to ensure that Excel file being uploaded by user does not have duplicate rows w.r.t. 2 particular columns.</p>
  
  <p>Example:</p>
  
  <p>In the snippet below, I want to flag out that row 1 and 2 contain duplicate combination of <code>COMPANY_CODE</code> and <code>CLERK_CODE</code>:</p>
  
  <p><a href="https://i.stack.imgur.com/MaVVq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MaVVq.png" alt="enter image description here"></a></p>
  
  <p>If such duplicate combination is found I want to reject the entire file being imported and let the user know where the problem is.</p>
  
  <p>Any clues?</p>