bug#23123: 25.0.92; why two timers for midnight-mode?

[Leo Liu]

1. emacs -q
2. (midnight-mode 1)
3. C-h v timer-list

([nil 22263 45044 442656 nil undo-auto--boundary-timer nil nil 0]
 [nil 22264 4496 995549 86400 run-hooks
      (midnight-hook)
      nil 0]
 [nil 22264 4496 995549 86400 run-hooks
      (midnight-hook)
      nil 0])

There are two timers added by midnight. Bug?

Leo