BUG: CategoricalIndex allows reindexing with non-unique CategoricalIndex
`CategoricalIndex.reindex()` throws a `ValueError` if the target index is non-unique *and* not also a `CategoricalIndex`. This means a non-unique `CategoricalIndex` can be used to reindex, resulting in duplicated data:
```
In [1]: import pandas as pd
        categorical_index = pd.CategoricalIndex(['bar', 'foo', 'bar', 'foo'], ['bar', 'foo'])
        s = pd.Series(range(4), index=categorical_index)

In [2]: s
Out[2]: bar    0
        foo    1
        bar    2
        foo    3
        dtype: int64

In [3]: s.reindex(categorical_index.as_ordered())
Out[3]: bar    0
        bar    2
        foo    1
        foo    3
        bar    0
        bar    2
        foo    1
        foo    3
        dtype: int64
```

This PR removes the extraneous type check and adds tests checking that an exception is thrown for non-unique target indexes, regardless of type.

- [ ] closes #21809
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @qwhelan! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/category.py`](https://github.com/qwhelan/pandas/blob/6450827831a06f1d67ba9b9a6a8b2d3d7c03778b/pandas/core/indexes/category.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_analytics.py`](https://github.com/qwhelan/pandas/blob/6450827831a06f1d67ba9b9a6a8b2d3d7c03778b/pandas/tests/frame/test_analytics.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/test_category.py`](https://github.com/qwhelan/pandas/blob/6450827831a06f1d67ba9b9a6a8b2d3d7c03778b/pandas/tests/indexes/test_category.py) !



##### Comment last updated on November 28, 2018 at 22:26 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23963?src=pr&el=h1) Report
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| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23963?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/23963/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.88% <100%> (ø)` | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23963?src=pr&el=footer). Last update [6dd130a...9550880](https://codecov.io/gh/pandas-dev/pandas/pull/23963?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback I stumbled across this bug while writing some `asv` benchmarks and only noticed #21809 after opening this PR, so it ended up being a few more lines to handle the `df.round()` case.

As we're now raising on non-unique `CategoricalIndex`es, we need to whitelist non-unique `target`s that are also the current index to allow `df.round()` to run. As the construction of the result involves a `.reindex()` call, we'd otherwise get `ValueError`s when calling `df.round()` on a non-uniquely indexed DataFrame.

This is already done in `Index.reindex()`, so this whitelisting is bringing `CategoricalIndex` back in line with the base implementation. Additionally, this is the fixed output from #21809:
```
print(dfb.round(3))
         a      b      c
low  0.087  0.230  0.411
low  0.311  0.566  0.545
low  0.807  0.918  0.522
hi   0.425  0.072  0.899
hi   0.421  0.582  0.214
hi   0.447  0.468  0.101
``` @jreback Changes implemented, the one failing test is a flaky hypothesis failure in an unrelated file. @jreback Green other than the unrelated flaky test thanks @qwhelan 