def normalize
    if defined?(@extra_rdoc_files) and @extra_rdoc_files then
      @extra_rdoc_files.uniq!
      @files ||= []
      @files.concat(@extra_rdoc_files)
    end

    @files            = @files.uniq if @files
    @extensions       = @extensions.uniq if @extensions
    @test_files       = @test_files.uniq if @test_files
    @executables      = @executables.uniq if @executables
    @extra_rdoc_files = @extra_rdoc_files.uniq if @extra_rdoc_files
  end
#
Normalize the list of files so that:
* All file lists have redundancies removed.
* Files referenced in the extra_rdoc_files are included in the package
file list.