title: Pandas: renaming columns that have the same name
body: <p>I have a dataframe that has duplicated column names a, b and b. I would like to rename the second b into c. </p>
  
  <pre><code>df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "b1": [7, 8, 9]})
  df.rename(index=str, columns={'b1' : 'b'})
  </code></pre>
  
  <p>Trying this with no success..</p>
  
  <pre><code>df.rename(index=str, columns={2 : "c"})
  </code></pre>