select c.*
from customer c join
     customer_product cp
     on c.customer_id = cp.customer_id join
     product p
     on p.product_serialnumber = cp.product_serialnumber
where p.product_id = @PRODUCT_ID;

select distinct c.*
from customer c join
     customer_product cp
     on c.customer_id = cp.customer_id join
     product p
     on p.product_serialnumber = cp.product_serialnumber
where p.product_id = @PRODUCT_ID;