Two countdown timers crash in same activity on android? [android] -

i have 2 activity. activitya has countdowntimer , when start activitya , timer starts (60 seconds).

in 60 seconds , start activityb , in activitya timer continue, because if don't in activityb, when 60 seconds finish, return activitya.

but 60 seconds before end , when return activitya, on create 1 more timer starts. previous timer , timer crashes. how prevent it?