nps – Rating Score: Should a user be allowed to rate as many times as he wants, and how to deal with that score?

I’m new to UX and just trying to make sure I ask the UX community for help on questions like this:

I’m working on a product that has a rate us functionality for NPS score. I’m wondering if

A – user should be allowed to rate again (elaboration – User has given 1 rating, and then goes back after some period of time and rates again)

B – How to cater for this score. Should I overwrite the previous rating response and only count their new rating. Or should I average their rating and make that count into NPS. Or should I consider this new rating as a new response (almost as if from a new user) and take that into account for NPS?

I would really appreciate some help as I’m a little stuck here.

Thank you