Fix DataFrame.LoadCsv can not load CSV with duplicate column names

[asmirnov82]

Fixes #6182

[codecov]

Codecov Report

Merging #6772 (2c72e8c) into main (39235a7) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6772      +/-   ##
==========================================
+ Coverage   69.03%   69.06%   +0.02%     
==========================================
  Files        1237     1237              
  Lines      252700   252742      +42     
  Branches    26446    26449       +3     
==========================================
+ Hits       174449   174544      +95     
+ Misses      71308    71261      -47     
+ Partials     6943     6937       -6     

Flag	Coverage Δ
Debug	69.06% <100.00%> (+0.02%)	⬆️
production	63.63% <100.00%> (+0.02%)	⬆️
test	88.84% <100.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
src/Microsoft.Data.Analysis/DataFrame.IO.cs	83.12% <100.00%> (+0.58%)	⬆️
...Microsoft.Data.Analysis.Tests/DataFrame.IOTests.cs	99.00% <100.00%> (+0.02%)	⬆️

... and 7 files with indirect coverage changes

[michaelgsharp]

/azp run

[azure-pipelines]

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