Error "Document already has a root node"

I have already gone through the sites Nokogiri XML Builder error -> "Document already has a root node" still getting error Here's is my Controller:

@data = [params[:firstname],params[:lastname],params[:middlename]
xml_file = Nokogiri::XML::Builder.new do |xml|
xml.p{ xml.text 'All fields are to be mandatorily filled.}
xml.PersonalInformation {
xml.firstname { xml.text @data[0] }
xml.lastname  {xml.text @data[1]  }
xml.middlename {xml.text @data[2]}
xml.p  { xml.text 'Personla Deatils'}
}

Is i am doing any wrong??