Creation of POTCAR.stripped.stripped etc files

The logic for creating stripped POTCAR files is buggy. The first time an entry with a POTCAR file is processed, we create a POTCAR.stripped file, which is what we want. However, if it is processed again, we will incorrectly create a stripped version of the already stripped version, i.e. a file named POTCAR.stripped.stripped, and so on, creating a new file with each processing with one more ".stripped" in the name. This results in the creation of redundant files.