This add-on to Zen Cart gives you a link in your admin Tools menu that will search all of your products and display any products where the Products Model number field is the same.  If it finds duplication, you will see the list with the Products ID, Products Model and Products Name.  The Products Model is a clickable link to that product so you can change the model number to something else.

Products without any model number are also listed, but not clickable.

This module can help you with other software programs - like Quickbooks and Google Merchant Center Feed that require unique model numbers for your products.

Works with 1.3.9, 1.5.0, 1.5.1, 1.5.2, 1.5.3, 1.5.4, 1.5.5