title: Find unique values in one column that have the same index in Pandas DataFrame
body: <p>I am trying to see what are the unique values corresponding to each index in my data frame. Assume I have the following dataframe:</p>
  
  <pre><code>        A   
  ind1    a   
  ind1    a   
  ind1    a   
  ind1    b   
  ind1    b   
  ind2    a 
  ind2    b 
  ind2    c 
  </code></pre>
  
  <p>How can I get a table of:</p>
  
  <pre><code>        index_related_uniqe
  ind1    a
  ind1    b
  ind2    a
  ind2    b
  ind2    c
  </code></pre>
  
  <p>I tried to use drop_duplicates, but it applies its removal to the whole column. </p>
  
  <p>For example <em>df.drop_duplicates('A', keep='first')</em> gives:</p>
  
  <pre><code>ind1    a   
  ind1    b   
  ind2    c 
  </code></pre>
  
  <p>Any idea is highly appreciated.</p>