Who says that it is allowed? It is however very easy to do as long as you have statutory health insurance. You just book an appointment with a doctor or a pharmacy that does vaccinations, give them your insurance card and get the vaccination for free. Again and again. Nobody will (or even can) check your previous vaccinations. All they can do is ask the patient.

After the vaccination they will bill the insurance, which can take months. Then someone or most likely some fraud detection system at the insurance company has to notice that something strange is going on. And since the vaccine is cheap and not a controlled substance it will probably take quite a few vaccinations to trigger a review. And even then they can not prevent further vaccinations. The only way would be to cancel your insurance entirely, which I am not even sure they are allowed to do.