Actually, Its depends upon the discretion of the Questioner to select which answer is best and he/she can change it any time (later or at the very moment). Same thing is applicable in the matter of 'thanks'. If I click twice on +button (thanks button) you will get a notification (may be notification like this 'someone thanked your answer') but there will be no increase in the count. For example, If I press once, the count will increase 0 to1 after a second and if I press again after one second, then it will revert to 0. You will get notification for the first action not for the second. If I press button third time, it will show +1. I dont know about the notification for the third action.