[{"id":1,"segment":"AAS","partner":{}},{"id":2,"segment":"AAS","partner":{}},{"id":3,"segment":"BBS","partner":{}},{"id":4,"segment":"CCS","partner":{}},{"id":5,"segment":"DDS","partner":{}}]

db.collection.aggregate([{$group:{_id:"$segment",field:{$first:"$$ROOT"}}},{$replaceWith:"$field"},{$unset:["id","_id"]}])