Implement unique+array parts of 24024
They have tests that go with them and are reasonably self-contained.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=h1) Report
> Merging [#24527](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5283eadaf51af855d6aee0692dfa2ae91feb633?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `8.33%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24527/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24527      +/-   ##
==========================================
- Coverage   31.88%   31.88%   -0.01%     
==========================================
  Files         166      166              
  Lines       52413    52419       +6     
==========================================
  Hits        16714    16714              
- Misses      35699    35705       +6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.28% <8.33%> (-0.01%)` | :arrow_down: |
| #single | `31.88% <8.33%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/24527/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `38.09% <0%> (ø)` | :arrow_up: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/24527/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `26.45% <0%> (-0.37%)` | :arrow_down: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/24527/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `31.7% <50%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24527?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/24527?src=pr&el=footer). Last update [d5283ea...7018506](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=h1) Report
> Merging [#24527](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4b6be69df575edf36218dedd1869cdb6c46129e1?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `8.33%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24527/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24527      +/-   ##
==========================================
- Coverage   31.88%   31.88%   -0.01%     
==========================================
  Files         166      166              
  Lines       52413    52419       +6     
==========================================
  Hits        16714    16714              
- Misses      35699    35705       +6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.28% <8.33%> (-0.01%)` | :arrow_down: |
| #single | `31.88% <8.33%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/24527/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `38.09% <0%> (ø)` | :arrow_up: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/24527/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `26.45% <0%> (-0.37%)` | :arrow_down: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/24527/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `31.7% <50%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24527?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/24527?src=pr&el=footer). Last update [4b6be69...b7b7b67](https://codecov.io/gh/pandas-dev/pandas/pull/24527?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks