BUG: Fix issue with inserting duplicate columns in a dataframe (#14291)
- [x] closes #14291
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Resubmitting PR (@jorisvandenbossche)

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14431?src=pr) is 85.25% (diff: 100%)

> Merging [#14431](https://codecov.io/gh/pandas-dev/pandas/pull/14431?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14431   diff @@
==========================================
  Files           140        140          
  Lines         50655      50655          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43185      43185          
  Misses         7470       7470          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [233d51d...2698005](https://codecov.io/gh/pandas-dev/pandas/compare/233d51dd099f590379bd9144c40a61f6785b2b57...2698005dae0b37fa8a205371f799cf9e5069175d?src=pr)
 This looks good to me (restarted the failing mac build on travis, should be unrelated)
 @paul-mannino Can you rebase (there is a merge conflict, probably in the whatsnew file)
 thanks!
