title: Count duplicates in rows per column in pandas DataFrame
body: <p>I have a very long table like below:</p>
  
  <pre><code>    A    B    C    D    .......
  0   au   br   gt   uy
  1   cd   gq   gt   uy
  2   fg   br   gt   ml
  3   kl   br   gt   wx
  </code></pre>
  
  <p>..............</p>
  
  <p>I would like to count and to print duplicates per column like:</p>
  
  <pre><code>A   0    
  B   2     
  C   3     
  D   1    
  </code></pre>
  
  <p>I have only found to count duplicates for one column:</p>
  
  <pre><code>df.duplicated(['B']).sum()
  </code></pre>
  
  <p>Do I have to write all columns (about 30) or is it possible to use something from pandas? I have tried this but it doesn't work:</p>
  
  <pre><code>df.duplicated(df.loc[:,:]).sum()
  </code></pre>