at once vs once more

at once

adv
  • At the same time; simultaneously. 

  • Immediately; now; right away. 

once more

adv
  • Again, a further time, once again. 

  • Used other than figuratively or idiomatically: see once, more. 

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