title: Filtering out duplicate combinations in a data frame
body: <p>I have a data frame that looks as follows:</p>
  
  <pre><code>df &lt;- data.frame(x = c('a', 'b', 'c', 'd'), 
                   y = c('b', 'a', 'c', 'c'), 
                   z = c(1, 1, 1, 4))
  </code></pre>
  
  <p>I would like to identify duplicate cases/rows. Seems like simple task I thought I should be able to tackle via <code>duplicates</code> or <code>unique</code>. Not so much, unless I'm missing something obvious here.</p>
  
  <p>I would like to return a data frame where case 1 (a-b) and case 2 (b-a) are recognized as the same. In other words, the result of this should be</p>
  
  <pre><code>x y z
  a b 1
  c c 1
  d c 4
  </code></pre>
  
  <p>I don't care which case, 1 or 2, gets returned, so long as there is only one.</p>
  
  <p>Any help would be much appreciated!</p>