class Opal::Rewriters::Hashes::KeyDuplicatesRewriter::UniqKeysSet

def <<(element)
  if @set.include?(element)
    key, _ = *element
    key = element.type == :str ? key.inspect : ":#{key}"
    Kernel.warn "warning: key #{key} is duplicated and overwritten"
  else
    @set << element
  end
end

def initialize
  @set = Set.new
end