Martin, I am very new to this forum, but my suggestions are:
1) look at the database structures, the primary and foreign keys and their relationships (1-to-many, or many-to-many) of the two tables you are linking or using in your drop-down list. From what u listed, it seems that the Product has multiple single sales (Product Line);
2) try using the word "DISTINCT" in the select statement to obtain a single occurance of the Product Line, create a new table, New_Product_Line_Tbl. Then Display in the drop-down List, the Product and the New_Product_Line_Tbl, using the LEFT JOIN;
3) look at #1 and #2 outside of the drop-down-list, to see what is actually going on at this point.
Hope this helps, hard to say with out looking at the structure of your tables.