all at once

for this once

On this particular occasion, to make an exception

once again

Again, once more, one more time.

once and for all

Conclusively, finally, permanently.

once in a blue moon

Very rarely; very infrequently.

once in a purple moon

Extremely rarely; almost never.

once in a while

Occasionally; sometimes.

once or twice

A small, indefinite number of times.