[ChristineP]

What Next/Prev mod are you using? I just tested this with the Next/Prev mod that we purchased and it works fine without skipping categories.

You do not need to clone or add a product more than once when you want them displayed in more than one category. When you add or modify a product you will select a "Main category" and you can select "Additional categories" by holding down the Ctrl key for a PC or the Command key for a Mac. Doing this allows the product to display in more than one category without having dups.

Any database will have a Primary Key, whether it's set by someone managing the database or default by the database with a numeric count of items added. If you check in your phpMyAdmin you will see large table set that X-Cart has for your entire store database. If you look around in your database you will find what fields and tables that are set with a constraint of being "Primary Key". The queries that are already in place for the cart, call out certain fields to compile information for the store front, statistics, an order, an invoice, order history, export/import, etc. For any database it is a very bad idea to have duplicate (item) numbers if this is a key indicator for store items, because the information compiled will be garbage and you'll have hundreds of lines of dups. If your issue is with keeping a "root item number" for items that you have with different sizes, colors, etc., then you need to approach this differently with product variants.