duple vs indistinguishable

duple

adj
  • Double. 

  • Having two beats in each foot. 

  • Having two beats, or a multiple of two beats, in each measure. 

indistinguishable

adj
  • Not capable of being perceived or known. 

  • Not distinguishable; not capable of being perceived, known, or discriminated as separate and distinct 

noun
  • Any of a set of things that cannot be distinguished. 

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