perf: Rolling 'iter_lookbehind' breeze through duplicates

[ritchie46]

Adds a small branch that ensures we skip very expensive computations on duplicates.

closes #19912

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 59.38%. Comparing base (e9a66e3) to head (d7a3b13).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-time/src/windows/group_by.rs	66.66%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #19922       +/-   ##
===========================================
- Coverage   79.43%   59.38%   -20.06%     
===========================================
  Files        1554     1554               
  Lines      215609   215618        +9     
  Branches     2454     2454               
===========================================
- Hits       171272   128039    -43233     
- Misses      43778    87020    +43242     
  Partials      559      559               

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