If you export again with the same master material filename, it will overwrite your original master material file. If you don't want it to overwrite the file, rename it in PlayUp Preferences.
There is, however, a quirk with using this feature. Since there does not appear to be a way to specify a material file name for a given mesh to RC.EXE, your entities will appear in CryEngine with "Replace Me" textures. For example, when exporting these entities from SketchUp:
You will notice that they will come into CryEngine 3 like this:
You will need to select all of the objects, open your Material Browser, select the master MTL file, and click on button to apply the material set to the selected objects.
And that's it. It's an extra step, but it's pretty painless.
