Database
[{"fieldA":1,"fieldB":2},{"fieldA":2,fieldB:4},{"fieldA":1,"fieldB":2},{"fieldA":3,fieldB:4}]
Query
db.collection.aggregate([{"$group":{"_id":"$fieldA","count":{"$sum":1},fieldB:{$last:"$fieldB"}}},{"$match":{"_id":{"$ne":null},"count":{"$gt":1}}},{"$project":{"fieldA":"$_id","fieldB":1,"_id":0}}])