Editorial: Add an explicit operation for merging calendar field names

[anba]

"with duplicate elements removed" can be ambiguous, for example the result of «"a", "b", "a"» could either be «"a", "b"» or «"b", "a"».

[codecov]

Codecov Report

Merging #2294 (ca8677b) into main (0409774) will decrease coverage by 9.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2294      +/-   ##
==========================================
- Coverage   91.03%   81.84%   -9.19%     
==========================================
  Files          19       17       -2     
  Lines       10558    10539      -19     
  Branches     1695     1478     -217     
==========================================
- Hits         9611     8626     -985     
- Misses        934     1869     +935     
- Partials       13       44      +31     

Flag	Coverage Δ
test262	?
tests	81.84% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
polyfill/lib/plainyearmonth.mjs	43.36% <0.00%> (-32.66%)	⬇️
polyfill/lib/plaintime.mjs	37.82% <0.00%> (-31.38%)	⬇️
polyfill/lib/plainmonthday.mjs	51.06% <0.00%> (-31.21%)	⬇️
polyfill/lib/plaindate.mjs	56.06% <0.00%> (-26.23%)	⬇️
polyfill/lib/plaindatetime.mjs	53.26% <0.00%> (-25.06%)	⬇️
polyfill/lib/instant.mjs	76.11% <0.00%> (-14.93%)	⬇️
polyfill/lib/zoneddatetime.mjs	87.14% <0.00%> (-11.10%)	⬇️
polyfill/lib/duration.mjs	69.93% <0.00%> (-10.68%)	⬇️
polyfill/lib/now.mjs	91.22% <0.00%> (-8.78%)	⬇️
polyfill/lib/timezone.mjs	85.35% <0.00%> (-8.29%)	⬇️
... and 6 more

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[ptomato]

Good catch, thanks.