title: Check whether two indicators are the same
body: <p>I am given a large data table with two indicators <code>ind1</code> and <code>ind2</code> with possible repetitions. E.g.</p>
  
  <pre><code> set.seed(1)
   ind1 &lt;- sample(1:3,1000, replace=TRUE )
   ind2 &lt;- c("a","b","c")[ind1]
  
   dt &lt;- data.table(ind1=ind1, ind2=ind2)
  </code></pre>
  
  <p>I would like now to check, whether These two indicators group the data the same way, i.e.</p>
  
  <p>two rows have the same indicator <code>ind1</code> if and only if they also the same indicator <code>ind2</code>. In the above example, this would be the case by construction.</p>