.:Consulta por Agrupación:.

[quimfv]

A!!!!! que tienes repeticiones que no te interesan, ahora lo entiendo. (Un diseño raro...)

Código MySQL:

SELECT sbc.products_id, 
             SUM(sbc.products_quantity) as Total
FROM (SELECT DISTINCT top2.products_id, 
                             tosh2.orders_id, 
                             top2.products_quantity
             FROM toc_orders_products top2 
                   INNER JOIN toc_orders_status_history tosh2
                            ON top2.orders_id = tosh2.orders_id 
             WHERE tosh2.orders_status_id IN (5,6,7)
                  AND top2.products_id = 20) as sbc
GROUP BY sbc.products_id;
 

La subconsulta da lo primero que pides

products_id | orders_id | products_quantity
---------------------------- ------------------------ -----------------------------
20 | 5 | 1
---------------------------- |---------------------------|------------------------------
20 | 4 | 4
---------------------------- |---------------------------|------------------------------

y luego se suma...

products_id | total
---------------------------- ------------------------
20 | 5 |
---------------------------- |---------------------------|