Fix duplicated to support multi-index columns.

[ueshin]

No description provided.

[codecov-io]

Codecov Report

Merging #803 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
- Coverage    94.3%   94.29%   -0.01%     
==========================================
  Files          32       32              
  Lines        5828     5858      +30     
==========================================
+ Hits         5496     5524      +28     
- Misses        332      334       +2

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.64% <100%> (-0.15%)	⬇️
databricks/koalas/missing/frame.py	100% <0%> (ø)	⬆️
databricks/koalas/series.py	95.17% <0%> (ø)	⬆️
databricks/koalas/namespace.py	86.27% <0%> (+0.13%)	⬆️
databricks/koalas/typedef.py	83.75% <0%> (+0.62%)	⬆️

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[HyukjinKwon]

haven't taken a close look but seems fine.