Dear Pokh "Once" means "at one time - in the past" However, "at once" means "immediately" So you do need to take care.. - People were once likely to die of diptheria, pneumonia, and so on [= At one time, they were likely to die] However... - People were at once likely to die of diptheria [= As soon as they got the disease, they could immediately die] In the same way..
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