title: ValueError: cannot reindex from a duplicate axis, for two columns of same dataframe
body: <p>I have the below mentioned data frame as df.</p>
  
  <pre><code>           A         B         C         D         E
  0     sarang  1.138380 -0.383898 -0.444186 -0.383898
  1    -1.1499 -0.469425 -1.455522 -0.789694 -1.455522
  2  -0.688895 -0.737444 -0.238886 -0.602263 -0.238886
  3   -1.02036  0.400622 -0.020911 -1.366732 -0.020911
  4    1.97958 -1.252052 -0.977561 -0.111656 -0.977561
  5  0.0802027 -0.290397  1.672563  0.371790  1.672563
  </code></pre>
  
  <p>I am trying to concat the column C and D, into a new column 'E' using :</p>
  
  <pre><code>df['E']  = pd.concat([df['C'], df['D']])
  </code></pre>
  
  <p>The error it gives me is :</p>
  
  <pre><code>ValueError: cannot reindex from a duplicate axis
  </code></pre>
  
  <p>Also I if use the ignore_index command,</p>
  
  <pre><code>df['E']  = pd.concat([df['C'], df['D']], ignore_index = True)
  </code></pre>
  
  <p>Then I get the below output, which I'm unable to understand at all.</p>
  
  <pre><code>           A         B         C         D         E
  0     sarang  1.138380 -0.383898 -0.444186 -0.383898
  1    -1.1499 -0.469425 -1.455522 -0.789694 -1.455522
  2  -0.688895 -0.737444 -0.238886 -0.602263 -0.238886
  3   -1.02036  0.400622 -0.020911 -1.366732 -0.020911
  4    1.97958 -1.252052 -0.977561 -0.111656 -0.977561
  5  0.0802027 -0.290397  1.672563  0.371790  1.672563
  </code></pre>
  
  <p>What should I do otherwise to get rid of the error.</p>