produce true on distinct for less than 2 elements #7018

Job	Run time
build-mdbook	7s
docker-tests	20m 32s
compiler-warnings	2m 7s
format-check	4m 2s
scaladoc-check	2m 52s
integration-tests (ubuntu-latest, oopsla19)	18m 20s
unit-tests (ubuntu-latest)	5m 18s
integration-tests (ubuntu-latest, arrays)	8m 58s
unit-tests (macos-latest)	3m 41s
integration-tests (macos-latest, oopsla19)	17m 21s
	1h 23m 18s

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