once or twice

all at once

Unexpectedly; without warning; all of a sudden.

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.

think twice

To reconsider, use judgement; to proceed with caution or thought.