title: How to analyze all duplicate entries in this Pandas DataFrame?
body: <p>I'd like to be able to compute descriptive statistics on data in a Pandas DataFrame, but I only care about duplicated entries. For example, let's say I have the DataFrame created by:</p>
  
  <pre><code>import pandas as pd
  data={'key1':[1,2,3,1,2,3,2,2],'key2':[2,2,1,2,2,4,2,2],'data':[5,6,2,6,1,6,2,8]}
  frame=pd.DataFrame(data,columns=['key1','key2','data'])
  print frame
  
  
       key1  key2  data
  0     1     2     5
  1     2     2     6
  2     3     1     2
  3     1     2     6
  4     2     2     1
  5     3     4     6
  6     2     2     2
  7     2     2     8
  </code></pre>
  
  <p>As you can see, rows 0,1,3,4,6, and 7 are all duplicates (using 'key1' and 'key2'. However, if I index this DataFrame like so:</p>
  
  <pre><code>frame[frame.duplicated(['key1','key2'])]
  </code></pre>
  
  <p>I get </p>
  
  <pre><code>   key1  key2  data
  3     1     2     6
  4     2     2     1
  6     2     2     2
  7     2     2     8
  </code></pre>
  
  <p>(i.e., the 1st and 2nd rows do not show up because they are not indexed to True by the duplicated method). </p>
  
  <p>That is my first problem. My second problems deals with how to extract the descriptive statistics from this information. Forgetting the missing duplicate for the moment, let's say I want to compute the .min() and .max() for the duplicate entries (so that I can get a range). I can use groupby and these methods on the groupby object like so:</p>
  
  <pre><code>a.groupby(['key1','key2']).min()
  </code></pre>
  
  <p>which gives </p>
  
  <pre><code>           key1  key2  data
  key1 key2                  
  1    2        1     2     6
  2    2        2     2     1
  </code></pre>
  
  <p>The data I want is obviously here, but what's the best way for me to extract it?  How do I index the resulting object to get what I want (which is the key1,key2,data info)?</p>