Database
[{dates:["2019-01-29","2019-01-29","2019-01-29","2019-01-29","2019-02-06","2019-02-06","2019-02-06","2019-02-08","2019-06-04","2019-06-25","2019-07-26","2019-08-15","2019-08-15"]}]
Query
db.collection.aggregate([{"$project":{"result":{$divide:[{$size:"$dates"},{$size:{$setUnion:["$dates"]}}]}}}])