title: pandas set columns based on values of the same column
body: <p>I have the following dataframe:</p>
  
  <pre><code>userid       date
    1        2010-01-03
    2        2009-01-04
    3        2004-02-03
    1        2007-01-01
  </code></pre>
  
  <p>I want to create a column that outputs true if there is another similar userid in another row with an earlier date. i.e.</p>
  
  <pre><code>userid       date        userid_seen
    1        2010-01-03        t
    2        2009-01-04        f
    3        2004-02-03        f
    1        2007-01-01        f  
  </code></pre>
  
  <p>How do I do it?</p>