title: How to select duplicate rows with pandas?
body: <p>I have a dataframe like this:</p>
  
  <pre><code>import pandas as pd
  dic = {'A':[100,200,250,300],
         'B':['ci','ci','po','pa'],
         'C':['s','t','p','w']}
  df = pd.DataFrame(dic)
  </code></pre>
  
  <p>My goal is to separate the row in 2 dataframes:</p>
  
  <ul>
  <li>df1 = contains all the rows that do not repeat values along column <code>B</code> (unque rows).</li>
  <li>df2 = containts only the rows who repeat themeselves.</li>
  </ul>
  
  <p>The result should look like this:</p>
  
  <pre><code>df1 =      A  B C         df2 =     A  B C
        0  250 po p               0  100 ci s 
        1  300 pa w               1  250 ci t
  </code></pre>
  
  <p>Note: </p>
  
  <ul>
  <li>the dataframes could be in general very big and have many values that repeat in column B so the answer should be as generic as possible
  
  <ul>
  <li>if there are no duplicates, df2 should be empty! all the results should be in df1</li>
  </ul></li>
  </ul>