title: Pandas - group by id and drop duplicate with threshold
body: <p>I have the following data:</p>
  
  <pre><code>userid itemid
    1       1
    1       1
    1       3
    1       4
    2       1
    2       2
    2       3
  </code></pre>
  
  <p>I want to drop userIDs who has viewed the same itemID more than or equal to twice. 
  For example, userid=1 has viewed itemid=1 twice, and thus I want to drop the entire record of userid=1. However, since userid=2 hasn't viewed the same item twice, I will leave userid=2 as it is.</p>
  
  <p>So I want my data to be like the following:</p>
  
  <pre><code>userid itemid
    2       1
    2       2
    2       3
  </code></pre>
  
  <p>Can someone help me?</p>
  
  <pre><code>import pandas as pd    
  df = pd.DataFrame({'userid':[1,1,1,1, 2,2,2],
                     'itemid':[1,1,3,4, 1,2,3] })
  </code></pre>