Maya's default material is called Lambert1, if an object is imported with the same name it'll rename it so it can fit it in the scene. Your Lambert1 material is being renamed to Lambert2. Rename your Material something else before you export it to import it with the same name.
I'm not sure if there's a batch rename type of tool, but I'd say they easiest way to do it would be in the outliner. Use ctrl+LMB to rename. Hope that helps :)