[{"Empname":"Doug","Group":[{"Category":[{"Categoryid":123,"Categoryname":"science"},{"Categoryid":233,"Categoryname":"Maths"}]}]},{"Empname":"stark","Group":[{"Category":[{"Categoryid":123,"Categoryname":"science"},{"Categoryid":144,"Categoryname":"language "}]}]}]

db.collection.aggregate([{"$unwind":"$Group"},{"$unwind":"$Group.Category"},{"$group":{"_id":{"Categoryid":"$Group.Category.Categoryid","Categoryname":"$Group.Category.Categoryname",},"count":{"$sum":1}}},{"$match":{"count":{"$gt":1}}},{"$project":{"_id":0,"Categoryname":"$_id.Categoryname","count":1}}])