R how to remove first row of duplicate values from a big column : <p>in R I have a file (df) consisting in 2 very big columns, A and B (aprox. 1000000 elements each). I know I have many duplicate values in A. I know how to remove the duplicates (remove second rows of each duplicate):</p>

<pre><code>df1 = df[!duplicated(df$A), ] 
</code></pre>

<p>but I would like to remove the first rows in the duplicate and keep the second rows. For instance, in the following example, I would like to remove 71 T and keep 71 C, NOT the other way around:</p>

<pre><code>A   B

4   A
8   C
21  T
71  T
71  C
74  C
75  G
78  C
86  T
</code></pre>

<p>Thanks very much in advance</p>
