I don't fully comprehend that data scenario your dealing with, when you say you want to return one row, from what table do you want the row from, and what table is causing the multiple records? selecting a simple row from the db is not possible when you are grouping. You would have to aggregate everything you are trying to select. It sounds like you need to leverage your primary key on one of the tables and do a sub-select of some type.
select * from ProductsTable pt1
join (select left(pt1.ProductID, 13) as prodid, max(pt2.primarykey) as primarykey from ProductsTable pt2 group by left(pt1.ProductID, 13)) as a on a.primarykey=pt1.primarykey