title: Removing lines containing a unique first field with awk?
body: <p>Looking to print only lines that have a duplicate first field. e.g. from data that looks like this:</p>
  
  <pre><code>1 abcd
  1 efgh
  2 ijkl
  3 mnop
  4 qrst
  4 uvwx
  </code></pre>
  
  <p>Should print out:</p>
  
  <pre><code>1 abcd
  1 efgh
  4 qrst
  4 uvwx
  </code></pre>
  
  <p>(FYI - first field is not always 1 character long in my data)</p>