title: Remove rows that two columns have the same values by pandas
body: <p>Input：</p>
  
  <pre><code>    S   T   W      U
  0   A   A   1   Undirected
  1   A   B   0   Undirected
  2   A   C   1   Undirected
  3   B   A   0   Undirected
  4   B   B   1   Undirected
  5   B   C   1   Undirected
  6   C   A   1   Undirected
  7   C   B   1   Undirected
  8   C   C   1   Undirected
  </code></pre>
  
  <p>Output：</p>
  
  <pre><code>    S   T   W      U
  1   A   B   0   Undirected
  2   A   C   1   Undirected
  3   B   A   0   Undirected
  5   B   C   1   Undirected
  6   C   A   1   Undirected
  7   C   B   1   Undirected
  </code></pre>
  
  <p>For column S and T ,rows(0,4,8) have same values. I want to drop these rows.</p>
  
  <p>Trying:</p>
  
  <p>I used <code>df.drop_duplicates(['S','T']</code> but failed, how could I get the results.</p>