[{"id":1,"user":"Brian1","configs":["a","b","c","d"]},{"id":2,"user":"max_en","configs":["a","h","i","j"]},{"id":3,"user":"userX","configs":["t","u","s","b"]}]

db.collection.aggregate([{"$unwind":"$configs"},{$group:{_id:null,configs:{"$addToSet":"$configs"}}}])