A new custom material is created when applying existing material

I am trying to apply a material to a single surface of a few objects but every time I ‘assign to objects’
a new custom material is created for each instance. Am I doing something wrong or is there a way I can avoid this? For rendering purposes I need all of these surfaces to have the SAME material applied. Any help would be much appreciated!

pre select all the surfaces then right click the material and apply to selected.