title: sort and uniq a file with a specific column and only keep 1st value from given file
body: <p>I have sample file </p>
  
  <pre><code>$ cat a.csv
  a,1,c
  b,1,d
  d,3,a
  s,2,c
  a,3,s
  </code></pre>
  
  <p>Required </p>
  
  <pre><code>a,1,c
  s,2,c
  a,3,s
  </code></pre>
  
  <p>It must remove all other value after uniq but only keep 1st value</p>
  
  <p>sort and uniq </p>
  
  <pre><code>a,1,c
  s,2,c
  a,3,s
  </code></pre>
  
  <p>I try <code>sort -k2 -n a.csv</code> but gave me this result </p>
  
  <pre><code>a,1,c
  a,3,s
  b,1,d
  d,3,a
  s,2,c
  </code></pre>
  
  <p>when I try <code>sort -k2 -n a.csv  | uniq -d</code> I got blank result</p>