once again

all at once

Unexpectedly; without warning; all of a sudden.

come again

A polite formula used when one has not heard or understood what has been said, or to indicate surprise.

for this once

On this particular occasion, to make an exception

here we go again

An expression of frustration upon seeing or hearing something bad repeated.

now and again

Synonym of occasionally, sometimes, intermittently.

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.