[{_id:1,fields1:"a",fields2:1,fields3:"z"},{_id:2,fields1:"a",fields2:1,fields3:"z"},{_id:3,fields1:"f",fields2:2,fields3:"g"},{_id:4,fields1:"f",fields2:2,fields3:"g"},{_id:5,fields1:"j",fields2:3,fields3:"g"},]

db.collection.aggregate([{"$group":{"_id":{"fields1":"$fields1","fields2":"$fields2","fields3":"$fields3"},"duplicatesIds":{"$push":"$_id"}}},{"$match":{"$expr":{"$gt":[{"$size":"$duplicatesIds"},1]}}},{"$project":{"_id":{"$arrayElemAt":["$duplicatesIds",0]},"fields1":"$_id.fields1","fields2":"$_id.fields3","fields3":"$_id.fields2"}}])