duplicate definition names not detected in mutual block

[cppio]

Prerequisites

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• Check that your issue is not already filed:
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• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
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Steps to Reproduce

mutual

def foo := ()

def foo := ()

end

Expected behavior: An error saying that foo is defined twice.

Actual behavior: No error.

Versions

Lean 4.16.0-nightly-2025-01-18
Lean 4.0.0-m1

Impact

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