title: pandas to_excel() ignore/allow duplicate column names
body: <p>Is there way to ignore duplicate column names after using to_excel() function of pandas?</p>
  
  <p>Say, I have <strong>old_wb.xlsx</strong>:</p>
  
  <pre><code>&gt;&gt; df1 = pd.read_excel('wb1.xlsx')
          ---------------------merged header--------------------
          col1    col2   col3   col1   col4   col1   col2   col5
          test    test   test   test   test   test   test   test
  </code></pre>
  
  <p>and say I did <strong>some processing</strong> to my Excel file, for example, remove the merged header and save it to another Excel file:</p>
  
  <pre><code>&gt;&gt; df1.to_excel('new_wb.xlsx', 'Sheet1', merged_cells=False, header=None, index=False)
  </code></pre>
  
  <p>The column names of <strong>new_wb.xlsx</strong> looks like this:</p>
  
  <pre><code>        col1    col2   col3   col1.1   col4   col1.2   col2.1   col5
          test    test   test   test     test   test     test     test
  </code></pre>
  
  <p>It added <code>.1</code> to the duplicate column names and it also increments as the duplicate column name goes.</p>
  
  <p>I tried renaming the column name before using <code>to_excel()</code> but it didn't work. It seems the renaming of duplicates happens in <code>to_excel()</code>.</p>
  
  <pre><code>&gt;&gt; df1.rename(columns=lambda x: x.replace('.1',''))
  </code></pre>
  
  <p>Upon searching, I found an argument for <code>to_excel()</code> which is <code>mangle_dupe_cols=False</code>, unfortunately it returned:</p>
  
  <pre><code>ValueError: Setting mangle_dupe_cols=False is not supported yet
  </code></pre>
  
  <p>Any help on how to ignore duplicate column names in saving `to_excel()'</p>