A slot can only be sold once
Double-booking is the quiet failure of every hand-managed calendar: two people both message you for Saturday at 5 pm, you say yes to the first, and the second finds out days later. On NefyX the rule is enforced by the database itself, not by hoping two requests never arrive together. If two buyers try for the same slot in the same moment, exactly one succeeds and the other is told immediately that it was just taken and offered the next free time.
That guarantee holds under real load too. It is tested with simultaneous booking attempts against a real database, because a promise about concurrency that has only been tried one request at a time is not worth much.