Explicitly don't support Index.duplicated

[RainFung]

This PR proposes to explicitly unsupport Index.duplicated.

[RainFung]

@itholic Hello Lee, Can you give some advice?

[softagram-bot]

Softagram Impact Report for pull/1131 (head commit: 97b6700)

⚠️ Copy paste found

ℹ️ indexes.py: Copy paste fragment inside the same file on lines 762, 1133:

            raise NotImplementedError(
                \"Doesn't support symmetric_difference between Index & MultiIndex for now\")

        sdf_self = self._kdf._s...(truncated 477 chars)

Now that you are on the file, it would be easier to pay back some tech. debt.

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[itholic]

@RainFung Sure, let me take a look little after while :)

[HyukjinKwon]

I think we can just explicitly unsupport this API as its return type is NumPy arrow and the data size seems expected to be large.
We can just say users can easily work around by DataFrame.deduplicated() API.

>>> kdf.index.to_series().to_frame().duplicated()
0    False
1    False
2    False
3    False
4    False
5    False
6    False
7    False
8    False
9    False
Name: 0, dtype: bool

I think we shouldn't implement Series.duplicated() as well.

Can you refer #744 and fix it accordingly?

[ueshin]

oh wait.
Seems like Series.duplicated() returns Series. We can support it.
https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.duplicated.html

[HyukjinKwon]

Yeah, I missed it. @RainFung, can you implement this one?

[ueshin]

maybe we can implement it in a separate PR. just revert the updates for Series missing file.

[RainFung]

OK， i'll fix it today

[codecov-io]

Codecov Report

Merging #1131 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1131      +/-   ##
==========================================
+ Coverage   95.15%   95.19%   +0.04%     
==========================================
  Files          35       35              
  Lines        7017     7056      +39     
==========================================
+ Hits         6677     6717      +40     
+ Misses        340      339       -1

Impacted Files	Coverage Δ
databricks/koalas/missing/indexes.py	100% <ø> (ø)	⬆️
databricks/koalas/missing/common.py	100% <100%> (ø)	⬆️
databricks/koalas/missing/series.py	100% <100%> (ø)	⬆️
databricks/koalas/missing/groupby.py	100% <0%> (ø)	⬆️
databricks/koalas/frame.py	96.86% <0%> (+0.03%)	⬆️
databricks/koalas/groupby.py	91.61% <0%> (+0.22%)	⬆️
databricks/koalas/indexes.py	96.64% <0%> (+0.4%)	⬆️
databricks/koalas/indexing.py	94.57% <0%> (+0.41%)	⬆️

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[ueshin]

Did we made Series.duplicated deprecated?
What about #1131 (comment) ?