DISTINCT ON in an aggregate function in postgres

The most simple thing I discovered is to use DISTINCT over jsonb (not json!).
(jsonb_build_object creates jsonb objects)

SELECT 
   JSON_AGG(
       DISTINCT jsonb_build_object('tag_id', photo_tag.tag_id, 
                                  'name', tag.name)) AS tags
FROM photo
    LEFT OUTER JOIN comment ON comment.photo_id = photo.photo_id
    LEFT OUTER JOIN photo_tag ON photo_tag.photo_id = photo.photo_id
    LEFT OUTER JOIN tag ON photo_tag.tag_id = tag.tag_id
GROUP BY photo.photo_id