BUG: Respect dups in reindexing CategoricalIndex
When the indexer is identical to the elements, we should still return duplicates when the indexer
contains duplicates.

Closes #17323.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=h1) Report
> Merging [#17355](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/473a7f3c186f6b0bfd9d3ce413fb627cf7a8f111?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17355/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17355      +/-   ##
==========================================
- Coverage   91.03%   91.02%   -0.02%     
==========================================
  Files         162      162              
  Lines       49567    49567              
==========================================
- Hits        45125    45116       -9     
- Misses       4442     4451       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.8% <100%> (ø)` | :arrow_up: |
| #single | `40.24% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `98.53% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=footer). Last update [473a7f3...83798b4](https://codecov.io/gh/pandas-dev/pandas/pull/17355?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Is there a test that covers the case of this equal index but *without* duplicates ? (so for which the special casing is done) 
Didn't directly see it in the diff, but might be it is somewhere in the file @jreback @jorisvandenbossche : I think I have addressed all of your requests.  The only remaining question revolves largely around we should in principle seed this test or leave it alone. thanks!