When we import a model into Create, all data from that model comes through, including all materials. After importing an updated model, duplicates of each material may come through and be saved in your project. There's no harm done but if done multiple times you may notice a bloated materials list. The following steps will help you clean up your project of duplicated materials. 

Navigate to the Asset Library and open the Assets in Use folder, then open the Materials folder. Here you'll see all materials currently in use in your project. 
Rename the materials that are associated with the model changes to something unique. These will be your master materials that you'll use in Create. This renaming helps determine the difference between the properly set up materials you would like to use and the new materials that have come through with the new import. 

Next, Navigate in the Asset Library to the Generated Project Asset's folder and find an old material duplicate. Right-click the old material and choose "Select Objects Using Material" to see if there are any objects in your scene using the old material. 
If objects become highlighted then we can right click the material again and choose "Replace Project Asset". This will bring up a separate window with the old material selected as the "Old Asset" and you can now select your master material you had renamed as your "New Asset" and click confirm.
This action will replace any object's material from using the old material to the renamed master material. 
You can now double check to make sure no object is using the old material by right-clicking and choosing "Select Objects Using Material". No objects should be selected and the author should be free to delete this old material from the project. 

Alternatively, you can handle multiple materials project-wide by navigating to the Generated Project Assets folder in your Asset Library and right clicking on it.

"Delete Unassigned Materials" will go through your projects assets to see what materials are not assigned and will simply delete them. The second option 

"Merge Identical Materials" will go through each material and merge materials together so long as their properties are the exact same. 

This process of learning to find, replace, and delete existing materials in the scene can become essential depending on workflow and model iteration.