Gem::Package#verify_files

# File tmp/rubies/ruby-3.4.1/lib/rubygems/package.rb, line 686
def verify_files(gem)
  gem.each do |entry|
    verify_entry entry
  end

  unless @spec
    raise Gem::Package::FormatError.new "package metadata is missing", @gem
  end

  unless @files.include? "data.tar.gz"
    raise Gem::Package::FormatError.new \
      "package content (data.tar.gz) is missing", @gem
  end

  if (duplicates = @files.group_by {|f| f }.select {|_k,v| v.size > 1 }.map(&:first)) && duplicates.any?
    raise Gem::Security::Exception, "duplicate files in the package: (#{duplicates.map(&:inspect).join(", ")})"
  end
end