Flag class file collision as error #32778

Job	Run time
test_non_bootstrapped	0s
test	54m 14s
test_scala2_library_tasty	0s
test_windows_fast	31m 25s
MiMa	11m 50s
community_build_a	52m 8s
community_build_b	1h 29m 41s
test_windows_full	0s
community_build_c	1h 8m 47s
test_sbt	20m 18s
test_java8	0s
build-msi-package	0s
build-sdk-package / build	7m 54s
build-chocolatey-package / build	21s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 36m 38s

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