Are the phrases "once for all" and "once and for all" equally acceptable?
Solution 1:
Ngram shows a wider usage of once and for all and very little usage of once for all.
Once and for all:
Adv. once and for all - in a conclusive way; "we settled the problem conclusively"
Solution 2:
Once for all is now old fashioned, once and for all being standard. The Oxford English Dictionary from ca. thirty years ago says "once for all, now usually once and for all".