BUG/TST: Fix Excel writers with duplicated column names.
Test and fix for issue #5235

@jreback Added comment with issue number.
 gr8....also need a release notes entry.....thanks...then can merge
 @jreback Added the following to `release.rst`:

``` rst

  - Fixed bug in Excel writers where frames with duplicate column names weren't
    written correctly. (:issue `5235`)
```

If you wish to reword let me know and I will amend.
 @jreback  Fixed `:issue:` typo.
 gr8 waiting for Travis

then will merge
 thank you for the PR!
