Oh, ideally you wouldn't vaccinate people who are already immune, but saying it hinders herd immunity is a bit silly.
I find it's always helpful to do thought experiments at the limit.
Do you think reaching herd immunity requires more doses A) in a population that is 99% of the way to herd immunity, but that 25% of doses will be wasted because they'll be given to people who are already immune, or B) in a population that is 1% of the way to herd immunity, but only 4% of doses will be wasted in this way?