fix: distinct() now preserves order in corner cases

[krlmlr]

Closes #77.

[codecov-commenter]

Codecov Report

Merging #78 (837318b) into main (1c12f30) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 837318b differs from pull request most recent head 4af7c41. Consider uploading reports for the commit 4af7c41 to get more accurate results

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@@          Coverage Diff          @@
##            main     #78   +/-   ##
=====================================
  Coverage   7.56%   7.57%           
=====================================
  Files        121     121           
  Lines      19112   19113    +1     
=====================================
+ Hits        1446    1447    +1     
  Misses     17666   17666           

Files	Coverage Δ
R/distinct.R	74.57% <100.00%> (+0.43%)	⬆️

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