[{"_id":"100","appts":[{"count":2,"date":"2020-08-07"},{"count":2,"date":"2020-08-07"}]},{"_id":"103","appts":[{"count":1,"date":"2020-08-07"},{"count":3,"date":"2020-08-07"},{"count":2,"date":"2020-08-08"}]}]

db.collection.aggregate([{$unwind:"$appts"},{$group:{_id:{"_id":"$_id","date":"$appts.date"},"count":{$sum:"$appts.count"}}},{$group:{_id:"$_id._id","appts":{"$addToSet":{"count":"$count","date":"$_id.date"}}}}])