title: Identify groups of observations with identical values for sets of variables
body: <p>I have a <code>data.frame</code> like this:</p>
  <pre><code>example.df &lt;- data.frame(grp = rep(1:3, each = 2),
                           attr.a = c(1, 4, 1, 7, 4, 1),
                           attr.b = c(2, 5, 2, 8, 5, 2),
                           attr.c = c(3, 6, 3, 9, 6, 3))
  
  &gt; example.df
    grp attr.a attr.b attr.c
  1   1      1      2      3
  2   1      4      5      6
  3   2      1      2      3
  4   2      7      8      9
  5   3      4      5      6
  6   3      1      2      3
  </code></pre>
  <p>I want to identify if any groups (<code>grp</code>) contain identical sets of observations for <code>attr.a</code>, <code>attr.b</code> and <code>attr.c</code>. In the example, <code>grp</code> 1 &amp; 3 contain identical sets of observations (but, the sets of observations are not ordered in the same way). Also in the example, <code>grp</code> 2 contains only one set of observations that are identical to <code>grp</code> 1 &amp; 3. It should only be identified if all sets of observations are identical.</p>
  <p>It isn't important how the identical <code>grp</code>s are identified, as long as it is clear, but could look something like this:</p>
  <pre><code>  grp attr.a attr.b attr.c ident
  1   1      1      2      3     3
  2   1      4      5      6     3
  3   2      1      2      3     0
  4   2      7      8      9     0
  5   3      4      5      6     1
  6   3      1      2      3     1
  </code></pre>
  <p>Thanks!</p>