[{country:"USA",invoices:[{client:"Cliet_1",docNum:"23123j"},{client:"Cliet_2",docNum:"34123j"},{client:"Cliet_1",docNum:"3453412df"}]},{country:"Canada",invoices:[{client:"Cliet_3",docNum:"23123j"},{client:"Cliet_4",docNum:"34123j"},{client:"Cliet_4",docNum:"3453412df"}]},{country:"USA",invoices:[{client:"Cliet_1",docNum:"23123j"},{client:"Cliet_2",docNum:"34123j"},{client:"Cliet_5",docNum:"3453412df"}]}]

db.collection.aggregate([{"$group":{"_id":"$country","invoices":{"$addToSet":"$invoices.client"}}},{"$project":{"_id":0,"country":"$_id","clients":{"$reduce":{"input":"$invoices","initialValue":[],"in":{"$setUnion":["$$value","$$this"]}}}}}])