It isn’t a rule per se but it is generally expected as a standard of care that the provider should be monitoring the feet of diabetic patients. You can have decreased sensation to the monofilament and not be aware that you have the decreased sensation.
That being said, I agree with you, having it done four weeks apart is excessive, particularly when it was fine four weeks ago. The only thing I can think of is that the doctor didn’t chart it satisfactorily and they wanted to redo it but didn’t want to say so. Just a theory.