Gem::Package#verify_files

# File tmp/rubies/ruby-3.0.5/lib/rubygems/package.rb, line 664
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) and duplicates.any?
    raise Gem::Security::Exception, "duplicate files in the package: (#{duplicates.map(&:inspect).join(', ')})"
  end
end