[{"Id":"5","Group":[{"Name":"frank","Roll":"123"}]},{"Id":"6","Group":[{"Name":"John","Roll":"124"}]},{"Id":"7","Group":[{"Name":"John","Roll":"125"}]}]

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