I had the letter shortly after I’d had the first jab (booked after a phone call from the surgery.)
IMO it’s an insurance measure, since some people may have missed a call and don’t have an answer phone (or don’t check messages) or just have a mobile with poor reception, etc.
So nobody will be forgotten or fall through the net.
My letter did state quite clearly that if I’d already been contacted and booked by the GP surgery, the letter should be ignored.
So I don’t think it’s necessarily confusing, as long as people do actually read what they’re sent. Mine did come in several languages, too.