[{"observer":"machine1","seen":[{"page1":["/link1","/link3"]},{"page2":["/link4","/link1"]}],},{"observer":"machine2","seen":[{"page3":["/link2"]},{"page1":["/link5"]}],}]

db.collection.aggregate([{"$unwind":"$seen"},{"$addFields":{"seen":{"$objectToArray":"$seen"}}},{"$unwind":"$seen"},{"$unwind":"$seen.v"},{"$facet":{"pages":[{"$group":{"_id":"$observer","pages":{"$addToSet":"$seen.v"}}}],"links":[{"$group":{"_id":"$observer","pages":{"$addToSet":"$seen.k"}}}]}}])