I know a way on how you can fix issues when duplicating items. I'm here to provide you a few information and help you with this, gerdgoebel.
For now, the option to select no tax when duplicating an inventory assembly is unavailable. When duplicating an item, only items creates a duplicate without tax code. You'll want to manually change the new items to non-tax.
Also, if you duplicate a main (or "parent") item, the new item will become a parent item. If you duplicate a parent item that has subitems, only the parent item will be duplicated and not the subitems.
When duplicating a subitem, the new item will become a subitem. Lastly, if you duplicate an inventory item, all item information will be copied except "On Hand" and "Reorder Point." Those fields will also be set to "0."
Please read this article for more information: Add, Edit, And Delete Items.
For your future reference, you can run reports in QuickBooks Desktop so you'll be able to get the information you need. Here's an article for your guide: Understand Reports.
Keep in touch if there's anything else you need. I'll be right here if you need further assistance.