The rules at the time were that a birth had to be registered within 42 days at the Registry Office local to the place of birth. It was (and still is) an offence not to do so.
Now IF the family moved before the birth could be registered any other RO would permit a registration, at the discretion of the Cheif Registrar, BY DECLARATION. This could result in a dual entry in the index, but there should still only be ONE birth certificate. This was a very very unusual situation; as for it happenening more than once in the same family, I don't have enough time to type all the verys to say how unusual this would be. So buying the certificates is the only way to be sure.

Rob