Test that each differing method appears once in failure_message

The diff array contains a method twice when it differs between sides
(once from each set subtraction). `.uniq` prevents duplicate blocks
in the output; this test pins that behaviour.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thiagoa

test/duck_typer/interface_checker_test.rb

@@ -439,6 +439,15 @@ def test_failure_message_includes_differing_method_signatures
     assert_includes message, "foo()"
   end
 
+  def test_failure_message_shows_each_differing_method_once
+    left = Class.new { def foo(a) = nil }
+    right = Class.new { def foo = nil }
+
+    message = call_checker(left, right).failure_message
+
+    assert_equal 1, message.scan("foo(a)").size
+  end
+
   def test_failure_message_shows_not_defined_for_missing_method
     left = Class.new { def foo = nil; def bar = nil }
     right = Class.new { def foo = nil }