Explain it a second time without the sigh. The sigh is what they’ll remember.

The information in a repeated explanation is identical; the signal accompanying it is not. A sigh, a pause, a shift in tone tells the person that asking cost them something, and they will weigh that before asking again. The content is forgotten within a day and the impression of being an inconvenience is not.

When someone asks you the same thing twice, answer as though it were the first time. If your patience is genuinely gone, say you will come back to it rather than answering badly. The questions you are not asked are the expensive ones.