Database
[{"visitors":[[{"name":"matan","id":"61793e6a0e08cdcaf213c0b1"},{"name":"shani","id":"61793e910e08cdcaf213c0b5"}],[{"name":"david","id":"6179869cb4944c6b19b05a23"},{"name":"orit","id":"617986e535fdf4942ef659bd"}],[{"name":"david","id":"6179869cb4944c6b19b05a23"},{"name":"orit","id":"617986e535fdf4942ef659bd"}]]}]
Query
db.collection.aggregate([{"$unwind":"$visitors"},{"$unwind":"$visitors"},{"$group":{"_id":null,"visitors":{"$addToSet":{"id":"$visitors.id","name":"$visitors.name"}}}}])