[komal0000]

sort keeps only the last --check value, so a silent and a diagnosing check mode given together are not rejected:

$ sort --check=silent --check=diagnose-first check_fail.txt
sort: check_fail.txt:6: disorder: 5

GNU sort rejects this with sort: options '-cC' are incompatible (exit 2), as it already does for -c -C. The reverse case was wrong too: --check=quiet -C was rejected even though both modes are silent, and GNU accepts it.

With this change, --check collects all of its values (a bare -c/--check counts as diagnose-first). The check is silent if -C or any --check=silent|quiet is given, and diagnosing if any bare -c or --check=diagnose-first is given. Having both is rejected with the -cC error, and repeating the same mode is fine. I compared these cases against GNU sort 8.32:

args	GNU	before	after
--check=silent --check=diagnose-first	-cC incompatible, 2	disorder message, 1	-cC incompatible, 2
-c --check=quiet	-cC incompatible, 2	silent, 1	-cC incompatible, 2
--check=quiet -C	silent, 1	-cC incompatible, 2	silent, 1
--check=quiet --check=silent	silent, 1	silent, 1	silent, 1
--check=diagnose-first -c	disorder message, 1	disorder message, 1	disorder message, 1

-c=VALUE itself (with a short option) is still accepted. I left that out because it's clap's require_equals behavior for the shared -c/--check argument.

Fixes #15595

Tests: added test_check_repeated_modes. It fails on main (--check=silent --check=diagnose-first exits 1 with the disorder message) and passes with this change. cargo test --features sort --no-default-features -- test_sort:: gives 193 passed, 6 failed on Windows. The 6 failures (test_compress_fail, test_ext_sort_preserves_long_lines, test_merge_batch_size, test_merge_batches, test_output_is_input, test_unterminated_file_not_fused_across_chunk_boundary) fail the same way on main on my Windows machine. cargo fmt --check and cargo clippy -p uu_sort -- -D warnings are clean.

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[komal0000]

Added in ed025f5: ["-c=silent", "-c=diagnose-first"] is now in test_check_repeated_modes, so the exact case from the issue is covered. Thanks for the review!

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