doubleton vs duad

doubleton

noun
  • A set containing precisely two elements. 

  • A pair of cards of the same suit, which are the only cards of that suit in a player's hand 

duad

noun
  • An unordered pair. 

  • Dwadasama. 

  • A pair or couple. 

How often have the words doubleton and duad occurred in a corpus of books? (source: Google Ngram Viewer )