Do I need to duplicate all ITEM segment for a Trading Partner Brand?

The item segment is a different from the other segments. If you choose “Use Primary Brand” for the Item segment of a Document, the trading partner brand part number will go out, but everything else will be taken from the primary brand.

This was done so you don’t need to repeat everything in the item segment for the trading partner brand. You just need to supply the part numbers. Even if the part numbers are the same, you can use the item segment to limit parts to just those assigned to the trading partner.