Gem::SpecificationPolicy#validate_unique_links

# File tmp/rubies/ruby-3.3.0/lib/rubygems/specification_policy.rb, line 507
  def validate_unique_links
    links = @specification.metadata.slice(*METADATA_LINK_KEYS)
    grouped = links.group_by {|_key, uri| uri }
    grouped.each do |uri, copies|
      next unless copies.length > 1
      keys = copies.map(&:first).join("\n  ")
      warning <<~WARNING
        You have specified the uri:
          #{uri}
        for all of the following keys:
          #{keys}
        Only the first one will be shown on rubygems.org
      WARNING
    end
  end