[{"_id":1,"fruits":["Banana","Banana","Apple"]},{"_id":2,"fruits":["Apple"]}]

db.collection.aggregate([{$unwind:"$fruits"},{$group:{_id:{_id:"$_id",fruit:"$fruits"},count:{$sum:1}}},{$group:{_id:"$_id._id",fruits:{$push:{k:"$_id.fruit",v:"$count"}}}},{$set:{fruits:{$arrayToObject:"$fruits"}}}])