Unstable feature warnings are printed multiple times

When I run cargo fmt on stable the warnings about unstable features seem to be printed multiple times, one for each file that is being linted.

Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_doc_attributes = true`, unstable features are only available in nightly channel.
Warning: can't set `format_strings = true`, unstable features are only available in nightly channel.
Warning: can't set `merge_imports = true`, unstable features are only available in nightly channel.
Warning: can't set `reorder_impl_items = true`, unstable features are only available in nightly channel.
Warning: can't set `condense_wildcard_suffixes = true`, unstable features are only available in nightly channel.
Warning: can't set `unstable_features = true`, unstable features are only available in nightly channel.
Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_doc_attributes = true`, unstable features are only available in nightly channel.
Warning: can't set `format_strings = true`, unstable features are only available in nightly channel.
Warning: can't set `merge_imports = true`, unstable features are only available in nightly channel.
Warning: can't set `reorder_impl_items = true`, unstable features are only available in nightly channel.
...

Start by reproducing the issue with cargo fmt on stable in a project containing multiple Rust files and unstable feature settings. Trace where the warnings are emitted during per-file linting; done means each applicable unstable-feature warning is printed only once for the formatting run.