Fix no raise dup index when using drop with axis=0
- [x] closes #19186
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19230?src=pr&el=h1) Report
> Merging [#19230](https://codecov.io/gh/pandas-dev/pandas/pull/19230?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c271d4dd0b014c41bd91048fa6e8d33c5bf27b44?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19230/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19230?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19230      +/-   ##
==========================================
+ Coverage   91.56%   91.56%   +<.01%     
==========================================
  Files         148      148              
  Lines       48856    48858       +2     
==========================================
+ Hits        44733    44735       +2     
  Misses       4123     4123
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.93% <100%> (ø)` | :arrow_up: |
| #single | `41.69% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19230?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19230/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.91% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19230/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.46% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/19230/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.35% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19230?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19230?src=pr&el=footer). Last update [c271d4d...5776bf3](https://codecov.io/gh/pandas-dev/pandas/pull/19230?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback I updated with proposed changes thanks @aschade nice patch! keep em coming! Will do :)

On Wed, Jan 17, 2018 at 7:53 PM Jeff Reback <notifications@github.com>
wrote:

> thanks @aschade <https://github.com/aschade> nice patch! keep em coming!
>
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