Possibly. Since Fred and George thought it would have been a good deal scarier "if she hadn't said the same thing to every single other student in their Divination class" rather than "if she hadn't said the same thing every year/week/day," its weakly implied that this is the first time she's make this prediction.

edit: although "every single other student in their Divination class" is somewhat ambiguous and could mean that over the course of the semester she has made this prediction for everyone at different times.