The person asking again may be trying to protect a decision that depends on your answer.

Repeated questions rarely mean someone doubts you. More often they mean something is riding on the answer that you cannot see: a booking they have to make, a person they have to tell, money they cannot get back. From where you sit it looks like nagging. From where they sit it is the only way to avoid a mistake.

On a third asking, find out what it is for. The answer usually explains the persistence and often changes what you say. And when you are the one asking, say the reason out loud, because it turns you from someone who is pestering into someone who has a deadline.