Struct.new: check for duplicate members

spec/ruby/core/struct/new_spec.rb

@@ -74,6 +74,10 @@ def obj.to_str() "Foo" end
     end
   end
 
+  it "raises ArgumentError when there is a duplicate member" do
+    -> { Struct.new(:foo, :foo) }.should raise_error(ArgumentError, "duplicate member: foo")
+  end
+
   it "raises a TypeError if object is not a Symbol" do
     obj = mock(':ruby')
     def obj.to_sym() :ruby end
@@ -156,6 +160,10 @@ def platform
         obj.legs.should == 4
       end
 
+      it "raises when there is a duplicate member" do
+        -> { Struct.new(:foo, :foo, keyword_init: true) }.should raise_error(ArgumentError, "duplicate member: foo")
+      end
+
       describe "new class instantiation" do
         it "accepts arguments as hash as well" do
           obj = @struct_with_kwa.new({name: "elefant", legs: 4})

src/main/ruby/truffleruby/core/struct.rb

@@ -63,6 +63,11 @@ def self.new(klass_name, *attrs, keyword_init: true, &block)
       end
     end
 
+    duplicates = attrs.uniq!
+    if duplicates
+      raise ArgumentError, "duplicate member: #{duplicates.first}"
+    end
+
     klass = Class.new self do
       _specialize attrs