sameness vs monotony

sameness and monotony both are nouns.

Word	Noun	Adjective	Verb	Adverb
sameness	Yes	No	No	No
monotony	Yes	No	No	No

As nouns, sameness and monotony are synonyms defined as:

sameness and monotony: the quality of wearisome constancy, routine, and lack of variety

Other synonyms of sameness include humdrum.

sameness (noun)	monotony (noun)
the quality of being alike	the quality of wearisome constancy, routine, and lack of variety
the quality of wearisome constancy, routine, and lack of variety	constancy of tone or pitch or inflection