UPDATE public.mytable SET
jsonfieldname = jsonb_set( jsonfieldname, '{json_obj_key}', array_to_json(
	ARRAY(
		SELECT DISTINCT( UNNEST( ARRAY(
			SELECT json_array_elements_text( COALESCE( jsonfieldname::json->'json_obj_key', '[]' ) )
		) || ARRAY['Element to add'] ) )
	)
)::jsonb )
WHERE id = 23
RETURNING *;