you cant have the same name for multiple materials.
instead, you need to use the
same materialwhenever you make a new material its put in like a table of all the materials. doesn't matter if its on a model or not. (hence why you cant have the same names)
you can replace your materials with whatever here
say i wanna replace the yellow on my model with the red material i've already made, i just click this button and select red from the table of materials