title: Pandas join issue: columns overlap but no suffix specified
body: <p>I have the following data frames:</p>
  <pre><code>print(df_a)
       mukey  DI  PI
  0   100000  35  14
  1  1000005  44  14
  2  1000006  44  14
  3  1000007  43  13
  4  1000008  43  13
  
  print(df_b)
      mukey  niccdcd
  0  190236        4
  1  190237        6
  2  190238        7
  3  190239        4
  4  190240        7
  </code></pre>
  <p>When I try to join these data frames:</p>
  <pre><code>join_df = df_a.join(df_b, on='mukey', how='left')
  </code></pre>
  <p>I get the error:</p>
  <pre><code>*** ValueError: columns overlap but no suffix specified: Index([u'mukey'], dtype='object')
  </code></pre>
  <p>Why is this so? The data frames do have common <code>'mukey'</code> values.</p>