[REVIEW] Fix duplicate names issue in MultiIndex.deserialize

[galipremsagar]

Fixes: #9254

This PR fixes deserialize in cudf.MultiIndex so that there is no data-corruption happening when there are duplicate names.

[vyasr]

Fix looks good, one minor suggested improvement to the tests.

[codecov]

Codecov Report

Merging #9258 (d138cf5) into branch-21.10 (3ee3ecf) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head d138cf5 differs from pull request most recent head ce5f6a7. Consider uploading reports for the commit ce5f6a7 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.10    #9258      +/-   ##
================================================
- Coverage         10.85%   10.85%   -0.01%     
================================================
  Files               115      116       +1     
  Lines             19158    19168      +10     
================================================
  Hits               2080     2080              
- Misses            17078    17088      +10     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/_lib/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/multiindex.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/text.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/fuzzer.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/hash_vocab_utils.py	0.00% <0.00%> (ø)

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[galipremsagar]

@gpucibot merge