If it appears that the number is duplicated, it is because you have already started the registration at some earlier time. At this point, what you should do is log in and complete the registration steps (you should not try to register again).
If it appears that the number is duplicated, it is because you have already started the registration at some earlier time. At this point, what you should do is log in and complete the registration steps (you should not try to register again).