BUG: to_excel swaps order of values of duplicate columns
On master, as a small example:

```
In [1]: df = pd.DataFrame([[1,2,3,4],[5,6,7,8]], columns=['A','B','A','B'])

In [2]: df
Out[2]:
   A  B  A  B
0  1  2  3  4
1  5  6  7  8

In [4]: df.to_excel('test_excel_duplicate_columns.xlsx')
```

gives:

![capture](https://cloud.githubusercontent.com/assets/1020496/9700751/4cacf18e-540f-11e5-923b-e6d6689d569f.PNG)

So the values of columns 2 and 3 are swapped (not the column names)

BTW, this happens both with .xlsx as .xls (openpyxl / xlsxwriter / xlwt)

Possibly related: #10982, #10970
