I am confused on the SPE testing. I understand that it is for a fixed
physical problem and cannot be used for a progressive disorder or disease
but how often is it necessary to perform for a driver? Is it specific for
first time testing on a PE or is it used at every certification for that
driver? Thanks

A Skills Performance evaluation is valid for 2 years and can be renewed within 30 days of expiration.  If you are seeing a driver for a recertification in one year and he/she has a valid SPE certificate, you are good to go:

Skill Performance Evaluation