title: Concatenate duplicate values
body: <p>I have a table with some variables, say <code>var1</code> and <code>var2</code> and an identifier, and for some reasons, some identifiers have 2 observations.</p>
  
  <p>I would like to know if there is a simple way to put back the second observation of the same identifier into the first one, that is </p>
  
  <ul>
  <li>instead of having two observations, each with <code>var1</code> <code>var2</code> variables for the same identifier value</li>
  </ul>
  
  <pre>
  ID    var1    var2
  ------------------
  A1    12      13
  A1    43      53
  </pre>
  
  <ul>
  <li>having just one, but with something like <code>var1</code> <code>var2</code> <code>var1_2</code> <code>var2_2</code>. </li>
  </ul>
  
  <pre>
  ID    var1    var2    var1_2    var2_2
  --------------------------------------
  A1    12      13      43        53
  </pre>
  
  <p>I can probably do that with renaming all my variables, then merging the table with the renamed one and dropping duplicates, but I assume there must be a simpler version.</p>