implement variant duplication

lib/spree/core/product_duplicator.rb

@@ -28,6 +28,7 @@ module Spree
           new_product.updated_at = nil
           new_product.product_properties = reset_properties
           new_product.master = duplicate_master
+          new_product.variants = duplicate_variants
         end
       end
 
@@ -36,6 +37,12 @@ module Spree
         duplicate_variant(master)
       end
 
+      def duplicate_variants
+        product.variants.map do |variant|
+          duplicate_variant(variant)
+        end
+      end
+
       def duplicate_variant(variant)
         variant.dup.tap do |new_variant|
           new_variant.sku = ""