Database
[{"_id":ObjectId("5ebbf0e90cf269611e3d34a1"),"libelle":"Libelle for 2903","rncp":"RNCP_2903"},{"_id":ObjectId("5ebd39b90cf20c0ce837a51b"),"libelle":"Libelle for 2903","rncp":"RNCP_2903"},{"_id":ObjectId("5ebbcc0d0cf20c0ce8379819"),"libelle":"Libelle for 15027","rncp":"RNCP_15027"},{"_id":ObjectId("5eba930c0cf20c0ce83779bf"),"libelle":"Libelle for 15027","rncp":"RNCP_15027"}]
Query
db.collection.aggregate([{"$group":{"_id":"$rncp","libelle":{"$first":"$libelle"}}}])