TST/CLN: series.duplicated; parametrisation; fix warning
Splitting up #21645

Added tests for `duplicated`, parametrized two tests for `drop_duplicates`, fixed a warning from #21614.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21899?src=pr&el=h1) Report
> Merging [#21899](https://codecov.io/gh/pandas-dev/pandas/pull/21899?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2b51c968ca1e16a7fb517968576f8a9ab47ce1ed?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21899/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21899?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21899   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50323    50323           
=======================================
  Hits        46281    46281           
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.36% <ø> (ø)` | :arrow_up: |
| #single | `42.22% <ø> (-0.01%)` | :arrow_down: |


------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21899?src=pr&el=footer). Last update [2b51c96...e640040](https://codecov.io/gh/pandas-dev/pandas/pull/21899?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 needs a rebase @jreback All just left-overs from the previous module. Hopefully converging on a clean module soon... ;-) @h-vetinari absolutely - is just easier to fix when exracting things and make nice and clean > can u do the same for frame? (unique and duplicates)

@jreback I did the same for frame in #21898 @jreback Actually, did the same for #21900 as well - e.g. replace `pd.` with direct imports thanks @h-vetinari 