For users that read their own configuration files and/or choose PetscOptionsInsertFile after PetscInitialize, we don't have a good way to avoid overwriting PETSC_OPTIONS or command-line options. The user could manually find argv and the environment variable, but that's a poor abstraction. Should PetscOptionsInsertFile learn how to behave so as to add new entries to the options database, but not supersede any that already exist?