If I append an object with a material that already exist in my .blend Blender makes a new one anyway
An object in a new file hasn’t to have the same materials, although they have the same names.
I’ve jsut try this: Put the tree window, select the square representing the new material, and press ‘Mays+L’ twice; thus, all objects with that one, are selected in the 3d windows. Now select an object with the old material (the one you want to be for all the objects): this object is the active object now. Then press CTRL+L and select ‘Materials’. Be present that it links ALL materials in the original object, to the others…