Enforce EnumValue name uniqueness

Raises an exception when trying to add a value that already exists with
the same name.

lib/graphql/enum_type.rb

@@ -93,6 +93,10 @@ def values=(new_values)
 
     # @param enum_value [EnumValue] A value to add to this type's set of values
     def add_value(enum_value)
+      if @values_by_name.key?(enum_value.name)
+        raise "Enum value names must be unique. `#{enum_value.name}` already exists."
+      end
+
       @values_by_name[enum_value.name] = enum_value
     end
 

spec/graphql/enum_type_spec.rb

@@ -110,4 +110,12 @@
       assert_equal(7, enum_2.values.size)
     end
   end
+
+  describe "validates enum value name uniqueness" do
+    it "raises an exception when adding a duplicate enum value name" do
+      assert_raises "Enum value names must be unique. `COW` already exists." do
+        enum.add_value(GraphQL::EnumType::EnumValue.define(name: "COW"))
+      end
+    end
+  end
 end