Implement ayufan validator2

app/validators/variable_duplicates_validator.rb

+# VariableDuplicatesValidator
+#
+# This validtor is designed for especially the following condition
+# - Use `accepts_nested_attributes_for :xxx` in a parent model
+# - Use `validates :xxx, uniqueness: { scope: :xxx_id }` in a child model
+class VariableDuplicatesValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    duplicates = value.reject(&:marked_for_destruction?).group_by(&:key).select { |_, v| v.many? }.map(&:first)
+    if duplicates.any?
+      record.errors.add(attribute, "Duplicate variables: #{duplicates.join(", ")}")
+    end
+  end
+end