It’s possible that people who have “gotten it twice” were people who tested positive, tested negative, then tested positive again, but at least one of those tests was false.
It’s also possible that the people who got it twice have difficulty “holding a titer,” and are also not immune to things they have been vaccinated for. Such people depend on herd immunity, which is one reason that people who refuse to vaccinate put other people in danger. Not holding a titer is an immune system dysfunction that is unusual, but not unheard of. If anyone ever told you they had chicken pox twice, they may not be lying or mistaken-- the latter is the most likely, but not being able to hold a titer is another possibility.
I know someone who is Deaf from rubella syndrome many, many years after the vaccine was invented. Her mother was vaccinated for it, but when she was born Deaf, and a long investigation showed that to be the reason, her mother had no antibodies. She was vaccinated again, and held antibodies for about 18 months, but they faded again. She was told if she ever planned to get pregnant again, to get vaccinated first, and hope the titer held until the baby was born (and I guess that it didn’t take too many months for her to become pregnant).
But some vaccines do fade in anyone. The current veterinary vaccine for rabies has to be repeated every 3 years (albeit, I have had vets tell me that after about four vaccines, most pets maintain permanent immunity, but the law in the US still requires vaccination every three years).
The adult (human) DTP shot has to be repeated every 10 years.
We may get a COVID shot that must be repeated, but perhaps it will not be yearly-- maybe it will be every three years, or five years.
If it’s five years, maybe there will be little clinics at the BMV. You don’t forget to renew your license, and they can prompt you to get your COVID booster.
Oh, and FWIW, vaccines don’t always contain a whole virus. Sometimes they just contain a portion of a protein coating on a virus. If it’s enough for the body to attack and disarm the virus, then it will work.