[{"name":"object1","subarray":[{"name":"subobject1","list":[{"date":"2020-01-01","name":"list1"}]}]}]

db.collection.aggregate([{"$match":{"name":"object1"}},{"$group":{"_id":"$name","list_array":{"$first":"$subarray.list"}}},{"$project":{list_array:{"$reduce":{"input":"$list_array","initialValue":[],"in":{"$setUnion":["$$this","$$value"]}}}}}])