question 106 Mobile vs. web

Hi There,

For the official solution,   Why there is no remove duplicates logic in the red part?   There will be duplicates in the session.  I think it should be

session AS (
    SEebLECT  session_id, user_id   

   FROM session_w

group by session_id, user_id 
    UNION ALL
    SELECT session_id, user_id
    FROM session_mobile

   group by session_id, user_id 
)

Thanks. 

WITH duration AS (
    SELECT 'web' as platform,  session_id,  duration
    FROM session_web_duration
    UNION ALL
    SELECT 'mobile' as platform, session_id, duration
    FROM session_mobile_duration
),
session AS (
    SEebLECT  session_id, user_id

    FROM session_w
    UNION ALL
    SELECT session_id, user_id
    FROM session_mobile
)
SELECT
    S.user_id,
    SUM(CASE WHEN D.platform='web' then duration ELSE 0 END) * 100.0 / SUM(duration )  AS web_duration,
    SUM(CASE WHEN D.platform='mobile' then duration ELSE 0 END) * 100.0 /SUM(duration )  AS mobile_duration,
    SUM(duration ) AS total_duration
FROM duration D
INNER JOIN session S
ON S.session_id = D.session_id
GROUP BY S.user_id;