adverb
simultaneously or suddenly
preposition
used to indicate that something happens or is done at the same time or in a very short period of time
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The emergency alarms went off all at once, signaling the need for immediate evacuation.
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I can't believe they all arrived at the party all at once, it was chaos!
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When the boss announced the surprise bonus, the whole team cheered all at once.
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The realization of their mistake hit them all at once, causing a wave of regret.