[{"items":["ObjectId1","ObjectId2","etc."]},{"items":["ObjectId1","ObjectId1","etc."]}]

db.collection.aggregate([{"$unwind":"$items"},{"$group":{"_id":"$_id","items_cnt":{"$sum":1},"unique_items":{"$addToSet":"$items"}}},{"$project":{"items_cnt":1,"unique_items_cnt":{"$size":"$unique_items"}}}])