[{"cust_id":1,"cust_name":"cust_1","uniqueKey":100},{"cust_id":2,"cust_name":"cust_2","uniqueKey":400},{"cust_id":3,"cust_name":"cust_3","uniqueKey":100},{"cust_id":4,"cust_name":"cust_4","uniqueKey":800},{"cust_id":5,"cust_name":"cust_5","uniqueKey":800}]

db.collection.aggregate([{"$group":{"_id":"$uniqueKey","count":{"$sum":1},items:{$push:{count:"$count",cust_name:"$cust_name"}}}},{"$match":{count:{"$gt":1}}},{$unwind:"$items"},{"$project":{_id:0,"uniqueKey":"$_id","cust_name":"$items.cust_name"}}])