Database
[{"_id":ObjectId("5a934e000102030405000001"),name:"Bob"},{"_id":ObjectId("5a934e000102030405000002"),name:"Alice"},{"_id":ObjectId("5a934e000102030405000003"),name:"Bob"},{"_id":ObjectId("5a934e000102030405000004"),name:"Alice"},{"_id":ObjectId("5a934e000102030405000005"),name:"Charlie"}]
Query
db.collection.aggregate([{$group:{_id:"$name",doc:{$last:"$$ROOT"}}},{$replaceRoot:{newRoot:"$doc"}}])