title: OpenPyXL error by saving of workbook: DefinedName with the same name and scope already exists
body: <p>I want edit one cell in a Excel-XLSX file with OpenPyXL in Python and then save this changed file into a xlsx with other name. 
  But i get the following error:
  Traceback (most recent call last):</p>
  
  <pre><code> File "E:\ExcelPythonTest\workspace\ExcelTest\ExcelTest.py", line 14, in &lt;module&gt;
      book.save(path2)
    File "C:\Python27\lib\site-packages\openpyxl-2.4.0b1-py2.7.egg\openpyxl\workbook\workbook.py", line 294, in save
      save_workbook(self, filename)
    File "C:\Python27\lib\site-packages\openpyxl-2.4.0b1-py2.7.egg\openpyxl\writer\excel.py", line 270, in save_workbook
      writer.save(filename)
    File "C:\Python27\lib\site-packages\openpyxl-2.4.0b1-py2.7.egg\openpyxl\writer\excel.py", line 251, in save
      self.write_data()
    File "C:\Python27\lib\site-packages\openpyxl-2.4.0b1-py2.7.egg\openpyxl\writer\excel.py", line 94, in write_data
      archive.writestr(ARC_WORKBOOK, write_workbook(self.workbook))
    File "C:\Python27\lib\site-packages\openpyxl-2.4.0b1-py2.7.egg\openpyxl\writer\workbook.py", line 121, in write_workbook
      defined_names.append(name)
    File "C:\Python27\lib\site-packages\openpyxl-2.4.0b1-py2.7.egg\openpyxl\workbook\defined_name.py", line 191, in append
      raise ValueError("""DefinedName with the same name and scope already exists""")
  ValueError: DefinedName with the same name and scope already exists
  </code></pre>
  
  <p>My Code:</p>
  
  <pre><code>import os
  import openpyxl
  
  # Change this to suit
  path = r'E:\ExcelPythonTest\example.xlsx'
  path2 = r'E:\ExcelPythonTest\examle_UHAHAHAHAHHA.xlsx'
  
  book = openpyxl.load_workbook( path )
  sheet = book.get_sheet_by_name('Dictionary')
  print sheet['G8'].value
  sheet['G8'] =  3.18
  print sheet['G8'].value
  
  book.save(path2)
  </code></pre>
  
  <p>Can anyone help me?</p>
  
  <p>P.S: i have tried to edit the same .xlsx with code in C and c-library libxl . So i think the xlsx file doen't have problem. </p>
  
  <p>I have tried the following code without any editing of xlsx:</p>
  
  <pre><code>import os
  import openpyxl
  
  # Change this to suit
  path = r'E:\ExcelPythonTest\Dt_RECORD_SHPS_RM_ROStruct.xlsx'
  path2 = r'E:\ExcelPythonTest\Dt_RECORD_SHPS_RM_ROStruct_UHAHAHAHAHHA.xlsx'
  
  book = openpyxl.load_workbook( path )
  
  book.save(path2)
  </code></pre>
  
  <p>But the same error was occurred.</p>