This may be hard to reproduce, or not even a bug, but here it goes:
Recently, I passed 100 reps in the r tag, and earned the corresponding bronze badge. However, a couple hours later, my rep in this tag went below 100 and then above 100. When I passed 100 reps for the second time, I did not receive a second badge (as expected); however, I received a second notification for earning the r badge.
Is this a bug?