again vs either

again

adv
  • Moreover; besides; further. 

  • Another time; once more. 

  • I ask again, I say again; used in repeating a question or statement. 

  • Over and above a factor of one. 

  • Back (to a former place or state). 

  • Here too, here also, in this case as well; used in applying a previously made point to a new instance; sometimes preceded by "here". 

  • Tell me again, say again; used in asking a question to which one may have already received an answer that one cannot remember. 

  • Used metalinguistically, with the repetition being in the discussion, or in the linguistic or pragmatic context of the discussion, rather than in the subject of discussion. 

either

adv
  • As well. 

pron
  • One or the other of two people or things. 

conj
  • Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by “or”. 

det
  • Any one (of two). 

  • Each of two; both. 

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