[rubygems/rubygems] Warn on duplicate dependency in a specification

https://github.com/rubygems/rubygems/commit/af3e5f7883

lib/rubygems/specification.rb

@@ -556,6 +556,10 @@ class Gem::Specification < Gem::BasicSpecification
   #   spec.add_runtime_dependency 'example', '~> 1.1', '>= 1.1.4'
 
   def add_runtime_dependency(gem, *requirements)
+    if requirements.uniq.size != requirements.size
+      warn "WARNING: duplicate dependency #{requirements}"
+    end
+
     add_dependency_with_type(gem, :runtime, requirements)
   end
 

test/rubygems/test_gem_specification.rb

@@ -3051,6 +3051,13 @@ Please report a bug if this causes problems.
     end
   end
 
+  def test_duplicate_runtime_dependency
+    expected = "WARNING: duplicate dependency [\"~> 3.0\", \"~> 3.0\"]\n"
+    assert_output nil, expected do
+      @a1.add_runtime_dependency "b", "~> 3.0", "~> 3.0"
+    end
+  end
+
   def set_orig(cls)
     s_cls = cls.singleton_class
     s_cls.send :alias_method, :orig_unresolved_deps , :unresolved_deps