What if there is more than one date duplicated?

eg. in B1 you've:
2017-05-12
2017-04-11
2017-04-05
2017-03-03
2017-03-02
2017-02-23
2017-02-23
2016-11-10
2016-11-10
2016-11-10
so what should be counted?
Do you've defined all dates in every cell?