title: Remove line if field is duplicate
body: <p>Looking for an awk (or sed) one-liner to remove lines from the output if the first field is a duplicate. </p>
  
  <p>An example for removing duplicate lines I've seen is:</p>
  
  <pre><code>awk 'a !~ $0; {a=$0}'
  </code></pre>
  
  <p>Tried using it for a basis with no luck (I thought changing the $0's to $1's would do the trick, but didn't seem to work).</p>