keyby= index when i subset is present too

[mattdowle]

Closes #3062

[codecov]

Codecov Report

Merging #3068 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3068      +/-   ##
==========================================
+ Coverage   90.85%   90.86%   +<.01%     
==========================================
  Files          61       61              
  Lines       11803    11807       +4     
==========================================
+ Hits        10724    10728       +4     
  Misses       1079     1079

Impacted Files	Coverage Δ
src/uniqlist.c	95.21% <100%> (+0.1%)	⬆️
R/data.table.R	91.71% <100%> (ø)	⬆️

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[renkun-ken]

Thanks for the fix! I'll re-run my production systems with latest dev and see if there are other issues.