My first thought isn't to answer your exact question but question why you would divide your products up into different tables based on manufacturer instead of just including Manufacturer as a field in your Product table? Are you also tracking other information about your Manufacturer in your Manufacturer table? If you keep all your Products in one table with Manufacturer just being another field in that table, you'd have the ability in any portal related to your Products table to sort products by type, manufacturer, etc. which you won't be able to do if the products are scattered across many tables.
As far as what you are asking though, have you used the Layout Manager to associate the layout you want with the Table you want? Sounds to me that your duplicated layout is still based on the original table and all the fields in it are still referencing that same table.
If you were to go into Layout Mode, then click Layout>Layout Setup, it calls up a window that allows you to associate the current layout with a particular table. Also accessible under Layout (when you're in Layout Mode) is the "Change Theme" window. Either of these might work for what you are actually trying to do.