Implement optimizer rule to eliminate double distinct

[ChrisJar]

Closes #925

[codecov-commenter]

Codecov Report

Merging #1008 (b464529) into main (7da66be) will increase coverage by 0.11%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1008      +/-   ##
==========================================
+ Coverage   80.47%   80.59%   +0.11%     
==========================================
  Files          75       75              
  Lines        4231     4231              
  Branches      765      765              
==========================================
+ Hits         3405     3410       +5     
+ Misses        662      653       -9     
- Partials      164      168       +4     

Impacted Files	Coverage Δ
dask_sql/_version.py	35.31% <0.00%> (+1.41%)	:arrow_up:

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