remove duplicated rows based on two columns : <p>I have got the following data.frame:</p>

<pre><code>df = read.table(text = 'a    b   c   d
                        1   12   2   1
                        1   13   2   1
                        1    3   3   1
                        2   12   6   2
                        2   11   2   2
                        2   14   2   2
                        1   12   1   2
                        1   13   2   2
                        2   11   4   3, header = TRUE')
</code></pre>

<p>I need to remove the rows which have the same observations based on columns <code>a</code> and <code>b</code>, so that the results would be:</p>

<pre><code>                        a    b   c   d
                        1   12   2   1
                        1   13   2   1
                        1    3   3   1
                        2   12   6   2
                        2   11   2   2
                        2   13   2   2
</code></pre>

<p>Thank you for any help</p>
